Current drive circuit and display device using same pixel circuit, and drive method

ABSTRACT

A display device including a current drive circuit capable of stably and correctly supplying an intended current to a light emitting element of each pixel without being affected by variations in characteristics of an active element inside the pixel and as a result capable of displaying a high quality image, wherein each pixel comprises a receiving use transistor TFT 3  for fetching a signal current Iw from a data line DATA when a scanning line SCAN-A is selected, a conversion use transistor TFT 1  for once converting a current level of a fetched signal current Iw to a voltage level and holding the same, and a drive use transistor TFT 2  for passing a drive current having a current level in accordance with the held voltage level through a light emitting element OLED. The conversion use thin film transistor TFT 1  generates a converted voltage level at its own gate by passing the signal current Iw fetched by the TFT 3  through its own channel. A capacitor C holds the voltage level created at the gate of the TFT 1 . The TFT 2  passes the drive current having a current level in accordance with the held voltage level through the light emitting element OLED.

TECHNICAL FIELD

[0001] The present invention relates to a current drive circuit fordriving an organic electroluminescence (EL) element or other lightemitting element controlled in brightness by a current, a display deviceproviding a light emitting element driven by this current drive circuitfor every pixel, a pixel circuit, and a method for driving a lightemitting element. In more detail, the present invention relates to acurrent drive circuit for controlling an amount of the current suppliedto a light emitting element by an insulating gate type field effecttransistor or other active element provided in each pixel and aso-called active matrix type image display device using the same.

BACKGROUND ART

[0002] In general, in an active matrix type image display device, animage is displayed by arranging a large number of pixels in a matrix andcontrolling a light intensity for every pixel in accordance with givenbrightness information. When using a liquid crystal as anelectro-optical substance, the transmittance of each pixel varies inaccordance with a voltage written into the pixel. In an active matrixtype image display device using an organic electroluminescence (EL)material as the electro-optical substance as well, the basic operationis similar to that of the case where a liquid crystal is used. However,unlike a liquid crystal display, an organic EL display is a so-calledself-luminescent type having a light emitting element for every pixel,so has the advantages of a better visual recognition of the image incomparison with a liquid crystal display, no need for back light, and afast response speed. The brightnesses of individual light emittingelements are controlled by the amount of current. Namely, this displayis largely different from a liquid crystal display in the point that thelight emitting elements are current driven types or current controlledtypes.

[0003] In the same way as a liquid crystal display, in an organic ELdisplay as well, there are a simple matrix and an active matrix drivemethods. The former is simple in structure, but makes it difficult torealize a large sized, high definition display, so the active matrixmethod is being vigorously developed. The active matrix method controlsthe current flowing through the light emitting element provided in eachpixel by an active element (generally a thin film transistor, one typeof the insulating gate type field effect transistor, hereinaftersometimes referred to as a “TFT”) provided inside the pixel. An organicEL display of this active matrix method is disclosed in for exampleJapanese Unexamined Patent Publication (Kokai) No. 8-234683. One pixel'sworth of an equivalent circuit is shown in FIG. 1. The pixel iscomprised of a light emitting element OLED, a first thin film transistorTFT1, a second thin film transistor TFT2, and a holding capacitor C. Thelight emitting element is an organic electroluminescence (EL) element.An organic EL element has a rectification property in many cases, so issometimes referred to as an OLED (organic light emitting diode). In thefigure, the symbol of a diode is used to indicate the light emittingelement OLED. However, the light emitting element is not always limitedto an OLED and may be any element controlled in brightness by the amountof the current flowing through it. Also, a rectification property is notalways required in the light emitting element. In the illustratedexample, a source of the TFT2 is set at a reference potential (groundpotential), an anode of the light emitting element OLED is connected toVdd (power supply potential), and a cathode is connected to a drain ofthe TFT2. On the other hand, a gate of the TFT1 is connected to ascanning line SCAN, the source is connected to a data line DATA, and thedrain is connected to the holding capacitor C and the gate of the TFT2.

[0004] In order to operate the pixel, first, when the scanning line SCANis brought to a selected state and a data potential Vw representing thebrightness information is applied to the data line DATA, the TFT1becomes conductive, the holding capacitor C is charged or discharged,and the gate potential of the TFT2 coincides with the data potential Vw.When the scanning line SCAN is brought to an unselected state, the TFT1becomes OFF and the TFT2 is electrically separated from the data lineDATA, but the gate potential of the TFT2 is stably held by the holdingcapacitor C. The current flowing through the light emitting element OLEDvia the TFT2 becomes a value in accordance with a gate/source voltageVgs, and the light emitting element OLED continuously emits the lightwith a brightness in accordance with the amount of the current suppliedthrough the TFT2.

[0005] When the current flowing between the drain and source of the TFT2is Ids, this is the drive current flowing through the OLED. Assumingthat the TFT2 operates in the saturated region, Ids is represented bythe following equation. $\begin{matrix}\begin{matrix}{{Ids} = {{\mu \cdot {Cox} \cdot {{W/L}/2}}( {{Vgs} - {Vth}} )^{2}}} \\{= {{\mu \cdot {Cox} \cdot {{W/L}/2}}( {{Vw} - {Vth}} )^{2}2}}\end{matrix} & (1)\end{matrix}$

[0006] Here, Cox is the gate capacity per unit area and is given by thefollowing equation:

Cox=∈0·∈r/d  (2)

[0007] In equation (1) and equation (2), Vth indicates a threshold valueof the TFT2, μ indicates a mobility of a carrier, W indicates a channelwidth, L indicates a channel length, ∈0 indicates a permittivity ofvacuum, ∈r indicates a dielectric constant of the gate insulating film,and d is a thickness of the gate insulating film.

[0008] According to equation (1), Ids can be controlled by the potentialVw written into the pixel. As a result, the brightness of the lightemitting element OLED can be controlled. Here, the reason for theoperation of the TFT2 in the saturated region is as follows. Namely,this is because, in the saturated region, Ids is controlled by only theVgs and does not depend upon the drain/source voltage Vds. Therefore,even if Vds fluctuates due to variations in the characteristics of theOLED, a predetermined amount of the drive current Ids can be passedthrough the OLED.

[0009] As mentioned above, in the circuit configuration of the pixelshown in FIG. 1, when written by Vw once, the OLED continues emittinglight with a constant brightness during one scanning cycle (one frame)until next rewritten. If large number of such pixels are arranged in amatrix as in FIG. 2, an active matrix type display device can beconfigured. As shown in FIG. 2, in a conventional display device,scanning lines SCAN-1 through SCAN-N for selecting pixels 25 in apredetermined scanning cycle (for example a frame cycle according to anNTSC standard) and data lines DATA giving brightness information (datapotential Vw) for driving the pixels 25 are arranged in a matrix. Thescanning lines SCAN-1 through SCAN-N are connected to a scanning linedrive circuit 21, while the data lines DATA are connected to a data linedrive circuit 22. By repeating the writing of Vw from the data linesDATA by the data line drive circuit 22 while successively selecting thescanning lines SCAN-1 through SCAN-N by the scanning line drive circuit21, an intended image can be displayed. In a simple matrix type displaydevice, the light emitting element contained in each pixel emits lightonly at an instant of selection. In contrast, in the active matrix typedisplay device shown in FIG. 2, the light emitting element of each pixel25 continues to emit light even after finishing being written.Therefore, in particular in a large sized, high definition display,there is the advantage that the level of the drive current of the lightemitting elements can be lowered in comparison with the simple matrixtype.

[0010]FIG. 3 schematically shows a sectional structure of the pixel 25shown in FIG. 2. Note, only OLED and TFT2 are represented forfacilitating the illustration. The OLED is configured by successivelysuperposing a transparent electrode 10, an organic EL layer 11, and ametal electrode 12. The transparent electrode 10 is separated for everypixel, acts as the anode of the OLED, and is made of a transparentconductive film for example ITO. The metal electrode 12 is commonlyconnected among pixels and acts as the cathode of the OLED. Namely, themetal electrode 12 is commonly connected to a predetermined power supplypotential Vdd. The organic EL layer 11 is a composite film obtained bysuperposing for example a positive hole transport layer and an electrontransport layer. For example, Diamyne is vapor deposited on thetransparent electrode 10 acting as the anode (positive hole injectionelectrode) as the positive hole transport layer, Alq3 is vapor depositedthereon as the electron transport layer. Further, a metal electrode 12acting as the cathode (electron injection electrode) is grown thereon.Note that, Alq3 represents 8-hydroxy quinoline aluminum. The OLED havingsuch a laminate structure is only one example. When a voltage in aforward direction (about 10V) is applied between the anode and thecathode of the OLED having such a configuration, injection of carrierssuch as electrons and positive holes occurs and luminescence isobserved. The operation of the OLED can be considered to be the emissionof light by excisions formed by the positive holes injected from thepositive hole transport layer and the electrons injected from theelectron transport layer.

[0011] On the other hand, the TFT2 comprises a gate electrode 2 formedon a substrate 1 made of glass or the like, a gate insulating film 3superimposed on the top surface thereof, and a semiconductor thin film 4superimposed above the gate electrode 2 via this gate insulating film 3.This semiconductor thin film 4 is made of for example a polycrystallinesilicon thin film. The TFT2 is provided with a source S, a channel Ch,and a drain D acting as a passage of the current supplied to the OLED.The channel Ch is located immediately directly above the gate electrode2. The TFT2 of this bottom gate structure is coated by an inter-layerinsulating film 5. A source electrode 6 and a drain electrode 7 areformed above that. Above them, the OLED mentioned above is grown viaanother inter-layer insulating film 9. Note that, in the example of FIG.3, the anode of the OLED is connected to the drain of the TFT2, so aP-channel thin film transistor is used as the TFT2.

[0012] In an active matrix type organic EL display, generally a TFT(thin film transistor) formed on a glass substrate is utilized as theactive element. This is for the following reason. Namely, an organic ELdisplay is a direct viewing type. Due to this, it becomes relativelylarge in size. Due to restrictions of cost and manufacturing facilities,a usage of a single crystalline silicon substrate for the formation ofthe active elements is not practical. Further, in order to extract thelight from the light emitting elements, usually a transparent conductivefilm of ITO (indium tin oxide) is used as the anode of the organic ELlayer, but ITO is frequently generally grown under a high temperaturewhich an organic EL layer cannot endure. In this case, it is necessaryto form the ITO before the formation of the organic EL layer.Accordingly, the manufacture process roughly becomes as follows:

[0013] Referring to FIG. 3 again, first the gate electrode 2, gateinsulating film 3, and semiconductor thin film 4 comprised of amorphoussilicon are successively stacked and patterned on the glass substrate 1to form the TFT2. In certain cases, the amorphous silicon is sometimesformed into polysilicon (polycrystalline silicon) by heat treatment suchas laser annealing. In this case, generally a TFT2 having a largerdegree of carrier mobility in comparison with amorphous silicon and alarger current driving capability can be formed. Next, an ITOtransparent electrode 10 acting as the anode of the light emittingelement OLED is formed. Subsequently, an organic EL layer 11 is stackedto form the light emitting element OLED. Finally, the metal electrode 12acting as the cathode of the light emitting element is formed by a metalmaterial (for example aluminum).

[0014] In this case, the extraction of the light is started from a backside (bottom surface side) of the substrate 1, so a transparent material(usually a glass) must be used for the substrate 1. In view of this, inan active matrix type organic EL display, a relatively large sized glasssubstrate 1 is used. As the active element, ordinarily use is made of aTFT as it can be relatively easily formed thereon. Recently, attemptshave also been made to extract the light from a front side (top surfaceside) of the substrate 1. The sectional structure in this case is shownin FIG. 4. The difference of this from FIG. 3 resides in that the lightemitting element OLED is comprised by successively superposing a metalelectrode 12 a, an organic EL layer 11, and a transparent electrode 10 aand an N-channel transistor is used as the TFT2.

[0015] In this case, the substrate 1 does not have to be transparentlike glass, but as the transistor formed on a large sized substrate, useis generally still made of a TFT. However, the amorphous silicon andpolysilicon used for the formation of the TFT have a worse crystallinityin comparison with single crystalline silicon and have a poorcontrollability of the conduction mechanism, therefore it has been knownthat there is a large variation in characteristics in formed TFTs.Particularly, when a polysilicon TFT is formed on a relatively largesized glass substrate, usually the laser annealing method is used asmentioned above in order to avoid the problem of thermal deformation ofthe glass substrate, but it is difficult to uniformly irradiate laserenergy to a large glass substrate. Occurrence of variations in the stateof the crystallization of the polysilicon according to the location inthe substrate cannot be avoided.

[0016] As a result, it is not rare for the Vth (threshold value) to varyaccording to pixel by several hundreds of mV, in certain cases, 1V ormore, even in the TFTs formed on an identical substrate. In this case,even if a same signal potential Vw is written with respect to forexample different pixels, the Vth will vary according to the pixels. Asa result, according to equation (1) described above, the current Idsflowing through the OLEDs will largely vary for every pixel andconsequently become completely off from the intended value, so a highquality of image cannot be expected as the display. A similar thing canbe said for not only the Vth, but also the variation of parameters ofequation (1) such as the carrier mobility μ. Further, a certain degreeof fluctuation in the above parameters is unavoidable not only due tothe variation among pixels as mentioned above, but also variations forevery manufacturing lot or every product. In such a case, it isnecessary to determine how the data line potential Vw should be set withrespect to the intended current Ids to be passed through the OLEDs forevery product in accordance with the final state of the parameters ofequation (1). Not only is this impractical in the mass productionprocess of displays, but it is also extremely difficult to devisecountermeasures for fluctuations in characteristics of the TFTs due tothe ambient temperature and changes of the TFT characteristics occurringdue to usage over a long period of time.

DISCLOSURE OF THE INVENTION

[0017] An object of the present invention is to provide a current drivecircuit capable of stably and accurately supplying an intended currentto a light emitting element etc. of a pixel without being affected byvariations in characteristics of an active element inside the pixel, adisplay device using the same and as a result capable of displaying ahigh quality image, a pixel circuit, and a method for driving a lightemitting element.

[0018] In order to achieve the object, the following means were devised.Namely, a display device according to the present invention provides ascanning line drive circuit for successively selecting scanning lines, adata line drive circuit including a current source for generating asignal current having a current level in accordance with brightnessinformation and successively supplying the same to data lines, and aplurality of pixels arranged at intersecting portions of the scanninglines and the data lines and including current driven type lightemitting elements emitting light by receiving the supply of the drivecurrent. The characterizing feature is that each pixel comprises areceiving part for fetching the signal current from the data line whenthe scanning line is selected, a converting part for converting acurrent level of the fetched signal current to a voltage level andholding the same, and a drive part for passing a drive current having acurrent level in accordance with the held voltage level through thelight emitting element. Specifically, the converting part includes aconversion use insulating gate type field effect transistor providedwith a gate, a source, a drain, and a channel and a capacitor connectedto the gate. The conversion use insulating gate type field effecttransistor generates a converted voltage level at the gate by passingthe signal current fetched by the receiving part through the channel.The capacitor holds the voltage level created at the gate. Further, theconverting part includes a switch use insulating gate type field effecttransistor inserted between the drain and the gate of the conversion useinsulating gate type field effect transistor. The switch use insulatinggate type field effect transistor becomes conductive when converting thecurrent level of the signal current to the voltage level andelectrically connects the drain and the gate of the conversion useinsulating gate type field effect transistor to create the voltage levelwith the source as the reference at the gate, while the switch useinsulating gate type field effect transistor is shut off when thecapacitor holds the voltage level and separates the gate of theconversion use insulating gate type field effect transistor and thecapacitor connected to this from the drain.

[0019] In one embodiment, the drive part includes a drive use insulatinggate type field effect transistor provided with a gate, a drain, asource, and a channel. This drive use insulating gate type field effecttransistor receives the voltage level held at the capacitor at its gateand passes a drive current having a current level in accordance withthat through the light emitting element via the channel. A currentmirror circuit is configured by direct connection of the gate of theconversion use insulating gate type field effect transistor and the gateof the drive use insulating gate type field effect transistor, whereby aproportional relationship is exhibited between the current level of thesignal current and the current level of the drive current. The drive useinsulating gate type field effect transistor is formed in the vicinityof the corresponding conversion use insulating gate type field effecttransistor inside the pixel and has an equivalent threshold voltage tothat of the conversion use insulating gate type field effect transistor.The drive use insulating gate type field effect transistor operates inthe saturated region and passes a drive current in accordance with adifference between the level of the voltage applied to the gate thereofand the threshold voltage through the light emitting element.

[0020] In another embodiment, the drive part shares the conversion useinsulating gate type field effect transistor together with theconverting part in a time division manner. The drive part separates theconversion use insulating gate type field effect transistor from thereceiving part and uses the same for driving after the conversion of thesignal current is completed and passes the drive current to the lightemitting element through the channel in a state where the held voltagelevel is applied to the gate of the conversion use insulating gate typefield effect transistor. The drive part has a controlling means forcutting off an unnecessary current flowing to the light emitting elementvia the conversion use insulating gate type field effect transistor attimes other than the time of drive. The controlling means cuts off theunnecessary current by controlling a voltage between terminals of a twoterminal type light emitting element having a rectification function.Alternatively, the controlling means comprises a control use insulatinggate type field effect transistor inserted between the conversion useinsulating gate type field effect transistor and the light emittingelement, and the control use insulating gate type field effecttransistor becomes nonconductive in state and separates the conversionuse insulating gate type field effect transistor and the light emittingelement when the light emitting element is not driven and switches tothe conductive state when the light emitting element is driven. Inaddition, the controlling means controls a ratio between a time forcutting off the drive current when the light emitting element is not tobe driven and placing the light emitting element in the non-lightemitting state and a time of passing the drive current when the lightemitting element is to be driven and placing the light emitting elementin the light emitting and thereby to enable the control of thebrightness of the pixel. According to a certain case, the drive part hasa potential fixing means for fixing the potential of the drain withreference to the source of the conversion use insulating gate type fieldeffect transistor in order to stabilize the current level of the drivecurrent flowing to the light emitting element through the conversion useinsulating gate type field effect transistor.

[0021] In a further developed form of the present invention, thereceiving part, the converting part, and the drive part configure acurrent circuit combining a plurality of insulating gate type fieldeffect transistors, and one or two or more insulating gate type fieldeffect transistors have a double gate structure for suppressing currentleakage in the current circuit. Further, the drive part includes theinsulating gate type field effect transistor provided with the gate,drain, and the source and passes the drive current passing between thedrain and the source to the light emitting element in accordance withthe level of the voltage applied to the gate, the light emitting elementis a two terminal type having an anode and a cathode, and the cathode isconnected to the drain. Alternatively, the drive part includes aninsulating gate type field effect transistor provided with a gate, adrain, and a source and passes a drive current passing between the drainand the source to the light emitting element in accordance with thelevel of the voltage applied to the gate, the light emitting element isa two terminal type having an anode and a cathode, and the anode isconnected to the source. Further, it includes an adjusting means fordownwardly adjusting the voltage level held by the converting part andsupplying the same to the drive part to tighten the black level of thebrightness of each pixel. In this case, the drive part includes aninsulating gate type field effect transistor having a gate, a drain, anda source, and the adjusting means downwardly adjusts the level of thevoltage applied to the gate by raising the bottom of the voltage betweenthe gate and the source of the insulating gate type field effecttransistor. Alternatively, the drive part includes an insulating gatetype field effect transistor having a gate, a drain, and a source, theconverting part is provided with a capacitor connected to the gate ofthe thin film transistor and holding the voltage level, and theadjusting means comprises an additional capacitor connected to thatcapacitor and downwardly adjusts the level of the voltage to be appliedto the gate of the insulating gate type field effect transistor held atthat capacitor. Alternatively, the drive part includes an insulatinggate type field effect transistor having a gate, a drain, and a source,the converting part is provided with a capacitor connected to the gateof the insulating gate type field effect transistor on its one end andholding the voltage level, and the adjusting means adjusts the potentialof the other end of the capacitor when holding the voltage levelconverted by the converting part at that capacitor to downwardly adjustthe level of the voltage to be applied to the gate of the insulatinggate type field effect transistor. Note that, as the light emittingelement, use is made of for example an organic electroluminescenceelement.

[0022] The pixel circuit of the present invention has the followingcharacteristic features. First, the brightness information is written toa pixel by passing a signal current having a magnitude in accordancewith the brightness through the data line. That current flows betweenthe source and the drain of the conversion use insulating gate typefield effect transistor inside the pixel and as a result creates avoltage between the gate and source in accordance with the currentlevel. Second, the voltage between the gate and source created asdescribed above or the gate potential is held by the function of thecapacitor formed inside the pixel or existing parasitically and is heldat about that level for a predetermined period even after the end of thewriting. Third, the current flowing through the OLED is controlled bythe conversion use insulating gate type field effect transistor per seconnected to it in series or the drive use insulating gate type fieldeffect transistor provided inside the pixel separately from that andhaving a gate commonly connected together with the conversion useinsulating gate type field effect transistor. The voltage between thegate and source at the OLED drive is generally equal to the voltagebetween the gate and source of the conversion use insulating gate typefield effect transistor created according to the first characterizingfeature. Fourth, at the time of writing, the data line and the internalportion of the pixel are made conductive by a fetch use insulating gatetype field effect transistor controlled by the first scanning line, andthe gate and the drain of the conversion use insulating gate type fieldeffect transistor are short-circuited by the switch use insulating gatetype field effect transistor controlled by the second scanning line.Summarizing the above,.while in the conventional example, the brightnessinformation was given in the form of a voltage value, in contrast, theremarkable characterizing feature of the display device of the presentinvention is that the brightness information is given in the form of acurrent value, that is, of a current written type.

[0023] As already mentioned, an object of the present invention is toaccurately pass the intended current through the OLEDs without beingaffected by variations in the characteristics of the TFTs. The reasonwhy the present object can be achieved by the first through fourthcharacterizing features will be explained below. Note that hereinafterthe conversion use insulating gate type field effect transistor will bedescribed as the TFT1, the drive use insulating gate type field effecttransistor will be described as the TFT2, the fetch use insulating gatetype field effect transistor will be described as the TFT3, and theswitch use insulating gate type field effect transistor will bedescribed as the TFT4. Note that the present invention is not limited toTFTs (thin film transistors). Insulating gate type field effecttransistors can be widely employed as the active elements, for example,single crystalline silicon transistors formed on a single crystallinesilicon substrate or SOI substrate. The signal current passing throughthe TFT1 at the time of writing of the brightness information is definedas Iw, and the voltage between the gate and source created in the TFT1as a result of this is defined as Vgs. At the time of writing, due tothe TFT4, the gate and the drain of the TFT1 are short-circuited, so theTFT1 operates in the saturated region. Accordingly, Iw is given by thefollowing equation.

Iw=μ1·Cox 1·W 1/L 1/2(Vgs−Vth 1)²  (3)

[0024] Here, the meanings of the parameters are similar to the case ofequation (1). Next, when defining the current flowing through an OLED asIdrv, Idrv is controlled in its current level by the TFT2 connected tothe OLED in series. In the present invention, the voltage between thegate and source thereof coincides with Vgs in equation (3). Therefore,when assuming that the TFT2 operates in the saturated region, thefollowing equation stands:

Idrv=μ2·Cox 2·W 2/L 2/2(Vgs−Vth 2)²  (4)

[0025] The meanings of the parameters are similar to the case ofequation (1). Note that, the condition for the operation of theinsulating gate type field effect transistor in the saturated region isgenerally given by the following equation while defining Vds as thevoltage between the drain and source.

|Vds|>|Vgs−Vth|  (5)

[0026] Here, TFT1 and TFT2 are formed close inside a small pixel, so itcan be considered that de facto μ1=μ2, Cox1=Cox2, and Vth1=Vth2. Then,at this time, the following equation is easily derived from equation (3)and equation (4):

Idrv/Iw=(W 2/L 2)/(W 1/L 1)  (6)

[0027] The point to be noted here resides in the fact that, in equation(3) and equation (4), the values of μ, Cox, and Vth per se vary forevery pixel, every product, or every manufacturing lot, but equation (6)does not include these parameters, so the value of Idrv/Iw is notaffected by such variation of them. For example, when designing W1=W2and L1=L2, Idrv/Iw=1 stands, that is, Iw and Idrv become an identicalvalue. Namely, the drive current Idrv flowing through the OLED becomesaccurately identical to the signal current Iw without being affected byvariations in the characteristics of the TFT. Therefore, as a result,the light emitting brightness of the OLED can be accurately controlled.The above description is just one example. As will be explained below bygiving embodiments, the ratio of Iw and Idrv can be freely determinedaccording to how W1, W2, L1, and L2 are set. Alternatively, it is alsopossible to use the same TFT for the TFT1 and TFT2.

[0028] In this way, according to the present invention, the correctcurrent can be passed through the OLED without being affected byvariations in the characteristics of the TFT. Further, according toequation (6), there is the large advantage of the simple proportionalrelationship between Iw and Idrv. Namely, in the conventional example ofFIG. 1, as shown in equation (1), Vw and Idrv are nonlinear and areaffected by variations in the characteristics of the TFT, so the controlof the voltage at the drive side becomes complex. Further, it is seenthat the carrier mobility μ among the characteristics of the TFT shownin equation (1) fluctuates according to the temperature. In this case,in the conventional example, according to equation (1), Idrv, andaccordingly the light emitting brightness of the OLED, changes, butaccording to the present invention, such a worry does not exist. Thevalue of Idrv given by equation (6) can be stably supplied to the OLED.

[0029] In equation (4), it was assumed that the TFT2 operated in thesaturated region, but the present invention is effective in also a casewhere the TFT2 operates in a linear region. Namely, where the TFT2operates in the linear region, Idrw is given by the following equation:

Idrv=μ2·Cox 2·W 2/L 2*{(Vgs−Vth 2)Vds 2−Vds 2 ^(2/2})  (7)

[0030] Vds2 is the voltage between the drain and source of TFT2. Here,when assuming that TFT1 and TFT2 are arranged close and as a resultVth1=Vth2=Vth stands, Vgs and Vth can be deleted from equation (3) andequation (7) and the following equation is obtained:

Idrv=μ2·Cox 2·W 2/L 2*{(2 Iw·L 1/μ1·Vds 2−Vds 2 ²/2}  (8)

[0031] In this case, the relationship between Iw and Idrv does notbecome a simple proportional relationship as in equation (6), but Vth isnot contained in equation (8). Therefore, it is seen that therelationship of Iw and Idrv is not affected by the variation of Vth(variation in a screen or variation for every manufacturing lot).Namely, by writing the predetermined Iw without being affected byvariation of the Vth, the intended Idrv can be obtained. Note, where μand Cox vary in the screen, due to these values, even if a specific Iwis given to the data line, the value of Idrv determined from equation(8) will vary. Therefore desirably the TFT2 operates in the saturatedregion as mentioned before.

[0032] Further, more preferably the TFT3 and the TFT4 are controlled bydifferent scanning lines, and the TFT4 is brought to the off statepreceding the TFT3 at the end of the write operation. In the pixelcircuit according to the present invention, the TFT3 and the TFT4 do nothave to be the same conductivity type. The pixel circuit may beconfigured so that the TFT3 and the TFT4 are an identical or differentconductivity types, the gates of them controlled by different scanninglines, and the TFT4 brought to the off state preceding to the TFT3 atthe end of the write operation.

[0033] Further, when the TFT3 and the TFT4 are controlled by differentscanning lines, after the end of the write operation, the TFT4 may bebrought to the on state by the operation of the scanning line, and thepixels extinguished in units of the scanning lines. This is because, thegate and the drain of the TFT1 and the gate of the TFT2 are connected,so the gate voltage of the TFT2 becomes the threshold value of the TFT1(this is almost equal to the threshold value of the TFT2), and both ofthe TFT1 and TFT2 become the off state.

[0034] In this way, by changing the timing of the extinguishing signal,it is possible to conveniently and freely change the brightness of thedisplay device. If the second scanning line is divided into colors of R,G, and B and separately controlled, adjustment of the color balance isalso easy.

[0035] Further, where it is desired to obtain the same time averagebrightness, the drive current of a light emitting element OLED can bemade larger by reducing the ratio of the light emitting period (duty).

BRIEF DESCRIPTION OF THE DRAWINGS

[0036]FIG. 1 is a circuit diagram of an example of a conventional pixelcircuit.

[0037]FIG. 2 is a block diagram of an example of the configuration of aconventional display device.

[0038]FIG. 3 is a sectional view of an example of the configuration of aconventional display device.

[0039]FIG. 4 is a sectional view of another example of the configurationof a conventional display device.

[0040]FIG. 5 is a circuit diagram of an embodiment of a pixel circuitaccording to the present invention.

[0041]FIG. 6 is a waveform diagram of an example of waveforms of signalsin the embodiment of FIG. 5.

[0042]FIG. 7 is a block diagram of an example of the configuration of adisplay device using a pixel circuit according to the embodiment of FIG.5.

[0043]FIG. 8 is a circuit diagram of a modification of the embodiment ofFIG. 5.

[0044]FIG. 9 is a circuit diagram of another embodiment of a pixelcircuit according to the present invention.

[0045]FIG. 10 is a waveform diagram of an example of the waveforms ofsignals in the embodiment of FIG. 9.

[0046]FIG. 11 is a circuit diagram of a modification of the embodimentof FIG. 9.

[0047]FIG. 12 is a circuit diagram of a modification of the embodimentof FIG. 9.

[0048]FIG. 13 is a circuit diagram of a modification of the embodimentof FIG. 9.

[0049]FIG. 14 is a circuit diagram of a modification of the embodimentof FIG. 9.

[0050]FIG. 15 is a circuit diagram of another embodiment of the pixelcircuit according to the present invention.

[0051]FIG. 16 is a circuit diagram of a modification of the embodimentof FIG. 15.

[0052]FIG. 17 is a circuit diagram of a modification of the embodimentof FIG. 15.

[0053]FIG. 18 is a circuit diagram of another embodiment of the pixelcircuit according to the present invention.

[0054]FIG. 19 is a circuit diagram of a modification of the embodimentof FIG. 18.

[0055]FIG. 20 is a view for explaining a case where the pixels areextinguished in units of scanning lines in the circuit of FIG. 19.

[0056]FIG. 21 is a circuit diagram of a modification of the embodimentof FIG. 19.

[0057]FIG. 22 is a circuit diagram of a modification of the embodimentof FIG. 19.

[0058]FIG. 23 is a diagram of characteristics of currents flowingthrough conversion use transistors of the circuit of FIG. 22 and theconventional circuit.

[0059]FIG. 24 is a circuit diagram of a modification of the embodimentof FIG. 19.

[0060]FIG. 25 is a view of data line potentials of the circuit of FIG.23 and the conventional circuit.

[0061]FIG. 26 is a circuit diagram of another embodiment of the pixelcircuit according to the present invention.

[0062]FIG. 27 is a circuit diagram of another embodiment of the pixelcircuit according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0063] Below, embodiments of the present invention will be explained byreferring to the attached drawings.

[0064]FIG. 5 shows an example of a pixel circuit according to thepresent invention. This circuit comprises, other than the conversion usetransistor TFT1 with the signal current flowing therethrough and thedrive use transistor TFT2 for controlling the drive current flowingthrough a light emitting element made of an organic EL element or thelike, a fetch use transistor TFT3 for connecting or disconnecting thepixel circuit and the data line DATA by the control of a first scanningline SCAN-A, a switch use transistor TFT4 for short-circuiting the gateand the drain of the TFT1 during the writing period by the control of asecond scanning line SCAN-B, a capacitor C for holding the voltagebetween the gate and source of the TFT1 even after the end of thewriting, and the light emitting element OLED. In FIG. 5, TFT3 isconfigured by a PMOS, and the other transistors are configured by NMOSs,but this is one example. The invention does not always have to be thisway. The capacitor C is connected to the gate of the TFT1 at its oneterminal, and connected to the GND (ground potential) at its otherterminal, but this is not limited to GND. Any constant potential ispossible. The anode of the OLED is connected to the positive powersupply potential Vdd.

[0065] Basically, the display device according to the present inventionis provided with a scanning line drive circuit for successivelyselecting scanning lines SCAN-A and SCAN-B, a data line drive circuitincluding a current source CS for generating a signal current Iw havinga current level in accordance with the brightness information andsuccessively supplying the same to the data lines DATA, and a pluralityof pixels arranged at intersecting portions of the scanning lines SCAN-Aand SCAN-B and data lines DATA and including current drive type lightemitting elements OLED emitting light by receiving the supply of thedrive current. As the characterizing feature, each pixel shown in FIG. 5comprises a receiving part for fetching the signal current Iw from thedata line DATA when the scanning line SCAN-A is selected, a convertingpart for once converting the current level of the fetched signal currentIw to the voltage level and holding the same, and a drive part forpassing the drive current having the current level in accordance withthe held voltage level through the light emitting element OLED.Specifically, the converting part includes a conversion use thin filmtransistor TFT1 provided with a gate, source, drain, and channel and thecapacitor C connected to the gate. The conversion use thin filmtransistor TFT1 generates a converted voltage level at the gate bypassing the signal current Iw fetched by the receiving part through thechannel, while the capacitor C holds the voltage level created at thegate. Further, the converting part includes the switch use thin filmtransistor TFT4 inserted between the drain and gate of the conversionuse thin film transistor TFT1. The switch use thin film transistor TFT4becomes conductive when converting the current level of the signalcurrent Iw to the voltage level, electrically connects the drain andgate of the conversion use thin film transistor TFT1, and creates thevoltage level with reference to the source at the gate of the TFT1.Further, the switch use thin film transistor TFT4 is cut off when thecapacitor C holds the voltage level and separates the gate of theconversion use thin film transistor TFT1 and the capacitor C connectedto this from the drain of the TFT1.

[0066] Further, the drive part includes a drive use thin film transistorTFT2 provided with a gate, drain, source, and channel. The drive usethin film transistor TFT2 receives the voltage level held at thecapacitor C at its gate and passes a drive current having a currentlevel in accordance with that via the channel to the light emittingelement OLED. A current mirror circuit is configured by directconnection of the gate of the conversion use thin film transistor TFT1and the gate of the drive use thin film transistor TFT2, whereby aproportional relationship is exhibited between the current level of thesignal current Iw and the current level of the drive current. The driveuse thin film transistor TFT2 is formed in the vicinity of thecorresponding conversion use thin film transistor TFT1 inside the pixeland has an equivalent threshold voltage to that of the conversion usethin film transistor TFT1. The drive use thin film transistor TFT2operates in the saturated region and passes a drive current inaccordance with the difference between the level of the voltage appliedto the gate thereof and the threshold voltage to the light emittingelement OLED.

[0067] The driving method of the present pixel circuit is as follows.The drive waveforms are shown in FIG. 6. First, at the time of writing,the first scanning line SCAN-A and the second scanning line SCAN-B arebrought into the selected state. In the example of FIG. 6, the firstscanning line SCAN-A is set at a low level, and the second scanning lineSCAN-B is set at a high level. By connecting the current source CS tothe data line DATA in a state where both scanning lines are selected,the signal current Iw in accordance with the brightness informationflows through the TFT1. The current source CS is a variable currentsource controlled in accordance with the brightness information. At thistime, the gate and the drain of the TFT1 are short-circuited by theTFT4, and therefore equation.(5) stands, and the TFT1 operates in thesaturated region. Accordingly, between the gate and the source thereof,a voltage Vgs given by equation (3) is created. Next, the first scanningline SCAN-A and the second scanning line SCAN-B are brought to theunselected state. In more detail, first the second scanning line SCAN-Bis set at a low level and the TFT4 is brought into an off state. Bythis, Vgs is held by the capacity C. Next, by setting the first scanningline SCAN-A at a high level and bringing it to the off state, the pixelcircuit and the data line DATA are electrically cut off, and therefore,the writing to the other pixel can be carried out via the data line DATAthereafter. Here, the data output by the current source CS as thecurrent level of the signal current must be effective at the point oftime when the second scanning line SCAN-B becomes unselected, but afterthat, may be set at any level (for example the write data of the nextpixel). The gate and the source of the TFT2 are commonly connectedtogether with the TFT1. Further, the two are formed close inside a smallpixel. Therefore, if the TFT2 operates in the saturated region, thecurrent flowing through the TFT2 is given by equation (4). This becomesthe drive current Idrv flowing through the light emitting element OLED.In order to operate the TFT2 in the saturated region, a sufficientpositive potential may be given to the Vdd so that equation (5) stillstands even if a voltage drop at the light emitting element OLED isconsidered.

[0068] According to the above drive, the current Idrv flowing throughthe light emitting element OLED is given by the previous equation (6):

Idrv=(W 2/L 2)/(W 1/L 1)·Iw

[0069] and a value correctly proportional to Iw without being affectedby variations in the characteristics of the TFT is obtained. Theproportional constant (W2/L2)/(W1/L1) can be set to a proper value byconsidering various circumstances. For example, where assuming that thevalue of the current to be passed through the light emitting elementOLED of one pixel is a relatively small value, for example 10 nA, as theactual problem, it is sometimes difficult to correctly supply such asmall current value as the signal current Iw. In such a case, if adesign is made so that (W2/L2)/(W1/L1)=1/100 stands, Iw becomes 1 μAfrom equation (6) and the current write operation becomes easy.

[0070] In the above example, it was assumed that the TFT2 operated inthe saturated region, but the present invention is effective even in thecase where the TFT2 operates in the linear region as mentioned before.Namely, where the TFT2 operates in the linear region, the current Idrvflowing through the light emitting element OLED is given by the aboveequation (8):

Idrv=μ2·Cox 2·W 2/L 2*{(2 Iw−L 1/μ1·Cox 1·W 1)^(1/2) Vds 2−Vds 2 ^(2/)2}

[0071] In the above equation, Vds2 is determined by current-voltagecharacteristics of the light emitting element OLED and the current Idrvflowing through the light emitting element OLED. When the potential ofVdd and the characteristics of the light emitting element OLED aregiven, it is a function of only Idrv. In this case, the relationshipbetween Iw and Idrv does not become the simple proportional relationshipas in equation (6), but if Iw is given, the Idrv satisfying equation (8)becomes the drive current flowing through the OLED. Vth is not containedin equation (8), therefore it is seen that the relationship between Iwand Idrv is not affected by the variation of Vth (variation for everypixel in the screen or variation for every manufacturing lot). Namely,by writing the predetermined Iw without being affected by variation inthe Vth, the intended Idrv can be obtained. In this way, when the TFT2operates in the linear region, the voltage between the drain and thesource of the TFT2 becomes small in comparison with the case where itoperates in the saturated region, therefore a low power consumption canbe realized.

[0072]FIG. 7 shows an example of the display device configured byarranging the pixel circuits of FIG. 5 in the matrix state. Theoperation thereof will be explained below. First, a vertical start pulse(VSP) is input to a scanning line drive circuit A21 including the shiftregister and a scanning line drive circuit B23 similarly including theshift register. After receiving VSP, the scanning line drive circuit A21and scanning line drive circuit B23 successively select first scanninglines SCAN-A1 to SCAN-AN and second scanning lines SCAN-B1 to SCAN-BNsynchronous to the vertical clocks (VCKA, VCKB). The current source CSis provided in the data line drive circuit 22 corresponding to each dataline DATA and drives the data line at a current level in accordance withthe brightness information. The current source CS comprises anillustrated voltage/current conversion circuit and outputs the signalcurrent in accordance with the voltage representing the brightnessinformation. The signal current flows through the pixel on the selectedscanning line, and the current is written in units of the scanninglines. Each pixel starts to emit light with an intensity in accordancewith its current level. Note, VCKA is slightly delayed relative to VCKBby a delay circuit 24. By this, as shown in FIG. 6, SCAN-B becomesunselected preceding SCAN-A.

[0073]FIG. 8 is a modification of the pixel circuit of FIG. 5. Thiscircuit gives a double gate configuration wherein two transistors TFT2 aand TFT2 b are connected in series to the TFT2 in FIG. 5 and imparts adouble gate configuration wherein two transistors TFT4 a and TFT4 b areconnected in series to the TFT4 in FIG. 5. The gates of the TFT2 a andTFT2 b and the gates of the TFT4 a and TFT4 b are commonly connected,therefore basically they perform a similar operation to that of singletransistors. As a result, also the pixel circuit of FIG. 8 performs asimilar operation to that of the pixel circuit of FIG. 5. With a singletransistor, particularly TFT, there is a case where the leakage currentat the off time becomes large according to a certain defect or the like.For this reason, when it is intended to suppress the leakage current,preferably a redundant configuration of connecting a plurality oftransistors in series is employed. This is because, when employing this,even if there is a leakage in one transistor, if the leakage of theother transistor is small, the leakage as a whole can be suppressed.When employing the configuration such as TFT2 a and TFT2 b of FIG. 8,due to the small leakage current, there arises a merit that the qualityof the black level of the display becomes good when the brightness iszero (current zero). Further, when employing the configuration such asTFT4 a and TFT4 b, there arises a merit that the brightness informationwritten in the capacitor C can be stably held. For these, similarly, itis also possible to configure three or more transistors in series. Asdescribed above, in the present modification, the receiving part,converting part, and the drive part configure the current circuitcombining a plurality of thin film transistors TFT. One or more thinfilm transistors (TFT) have the double gate structure for suppressingthe current leakage in the current circuit.

[0074]FIG. 9 shows another embodiment of the pixel circuit according tothe present invention. The characterizing feature of this circuitresides in that the transistor TFT1 with the signal current Iw flowingtherethrough per se controls the current Idrv flowing through the lightemitting element OLED. In the pixel circuit shown in FIG. 5 mentionedbefore, when the characteristics of TFT1 and TFT2 (Vth, μ or the like)are slightly different from each other, equation (6) does not correctlystand, and there is a possibility such that Iw and Idrv are notcorrectly proportional, but in the pixel circuit of FIG. 9, such aproblem does not occur in principle. The pixel circuit of FIG. 9 isprovided with, other than the TFT1, a transistor TFT3 for connecting ordisconnecting the pixel circuit and the data line DATA by the control ofthe first scanning line SCAN-A, a transistor TFT4 for short-circuitingthe gate and the drain of the TFT1 during the writing period by thecontrol of the second scanning line SCAN-B, a capacitor C for holdingthe voltage between the gate and source of the TFT1 even after the endof the writing, and a light emitting element OLED made of the organic ELelement. The holding capacitor C is connected to the gate of the TFT1 atits one terminal and connected to the GND (ground potential) at itsother terminal, but this is not limited to GND. Any constant potentialis possible. The anode of the light emitting element OLED is connectedto the anode line A arranged in units of the scanning lines. The TFT3 isconfigured by a PMOS, and the other transistors are configured by NMOSs,but this is one example. The invention does not always have to be thisway.

[0075] As described above, in the present embodiment, the drive part ofthe pixel circuit shares the conversion use thin film transistor TFT1 ina time division manner together with the conversion part. Namely, thedrive part separates the conversion use thin film transistor TFT1 fromthe receiving part after completing the conversion of the signal currentIw and uses the same for drive and passes the drive current to the lightemitting element OLED through the channel in the state where the heldvoltage level is applied to the gate of the conversion use thin filmtransistor TFT1. Further, the drive part has a controlling means forcutting off the unnecessary current flowing through the light emittingelement OLED via the conversion use thin film transistor TFT1 at timesother than the drive. In the case of the present example, thecontrolling means controls the voltage between terminals of the twoterminal type light emitting elements OLED having the rectificationfunction by the anode line A and cuts off the unnecessary current.

[0076] The driving method of this circuit is as follows. The drivewaveform is shown in FIG. 10. First, the first scanning line SCAN-A andthe second scanning line SCAN-B are brought to the selected state at thetime of writing. In the example of FIG. 10, the first scanning lineSCAN-A is set at a low level, and the second scanning line SCAN-B is setat a high level. Here, the current source CS of the current value Iw isconnected to the data line DATA, but in order to prevent the Iw fromflowing via the light emitting element OLED, the anode line A of thelight emitting element OLED is set at low level (for example GND ornegative potential) so that the light emitting element OLED becomes theoff state. By this, the signal current Iw flows through the TFT1. Atthis time, the gate and the drain of the TFT1 are electricallyshort-circuited by the TFT4, therefore equation (5) stands, and the TFT1operates in the saturated region. Accordingly, the voltage Vgs given byequation (3) is created between the gate and the source thereof. Next,the first scanning line SCAN-A and the second scanning line SCAN-B arebrought to the unselected state. In more detail, first, the secondscanning line SCAN-B is brought to the low level and the TFT4 is broughtto the off state. By this, the Vgs created in the TFT1 is held at thecapacity C. Next, by setting the SCAN-A at the high level and bringingthe TFT3 to the off state, the pixel circuit and the data line DATA areelectrically cut off, and therefore the writing to another pixel can becarried out via the data line DATA after that. Here, the data suppliedby the current source CS as the signal current Iw must be valid at apoint of time when the second scanning line SCAN-B becomes unselected,but may be set at any value (for example write data of the next pixel)after that. Then, the anode line A is brought to the high level. The Vgsof the TFT1 is held by the capacitor C, therefore if the TFT1 operatesin the saturated region, the current flowing through the TFT1 coincideswith Iw in equation (3). This becomes the drive current Idrv flowingthrough the light emitting element OLED. That is, the signal current Iwcoincides with the drive current Idrv of the light emitting elementOLED. In order to operate the TFT1 in the saturated region, a sufficientpositive potential may be given to the anode line A so that equation (5)still stands even if the voltage drop at the light emitting element OLEDis considered. According to the above drive, the current Idrv flowingthrough the light emitting element OLED correctly coincides with Iwwithout being affected by variations in the characteristics of the TFT.

[0077]FIG. 11 is a modification of the pixel circuit shown in FIG. 9. InFIG. 11, there is no anode line as in FIG. 9. The anode of the lightemitting element OLED is connected to the constant positive potentialVdd, while a P-channel transistor TFT5 is inserted between the drain ofthe TFT1 and the cathode of the light emitting element OLED. The gate ofthe TFT5 is controlled by the drive line drv arranged in units of thescanning lines. The object of insertion of TFT5 is prevention of theflow of the signal current Iw via the light emitting element OLED bysetting the drive line drv at a high level and bringing the TFT5 to theoff state at the time of writing data. After the writing is ended, thedrv is brought to the low level, the TFT5 is brought to the on state,and the drive current Idrv flows through the light emitting elementOLED. The rest of the operation is similar to that of the circuit ofFIG. 9.

[0078] The present example includes the TFT5 connected to the lightemitting element OLED in series and can cut off the current flowing tothe light emitting element OLED in accordance with the control signalgiven to the TFT5. The control signal is given to the gate of the TFT5included in each pixel on the identical scanning line via the drive linedrv provided in parallel to the scanning line SCAN. In the presentexample, the TFT5 is inserted between the light emitting element OLEDand the TFT1, and the current flowing through the light emitting elementOLED can be turned on or off by the control of the gate potential of theTFT5. According to the present example, the emission of light of eachpixel is achieved for the amount of time where the TFT5 is on by a lightemission control signal. When defining the on time as τ and the time ofone frame as T, the ratio in time when the pixel is emitting light, thatis, the duty, becomes approximately τ/T. A time average brightness ofthe light emitting element changes in proportional to this duty.Accordingly, by changing the on time τ by controlling the TFT5, it isalso possible to variably adjust the screen brightness of the EL displayconveniently and in a wide range.

[0079] As described above, in the present example, the controlling meanscomprises the control use thin film transistor TFT5 inserted between theconversion use thin film transistor TFT1 and the light emitting elementOLED. The control use thin film transistor TFT5 becomes nonconductiveand separates the conversion use thin film transistor TFT1 and the lightemitting element OLED when the light emitting element OLED is not drivenand switches to the conductive state at the time of drive. Further, thiscontrolling means can control the brightness of each pixel bycontrolling the ratio between the off time for which the drive currentis cut off and the light emitting element OLED is placed in thenon-light emitting state when the OLED is not to be driven and the ontime for which the drive current is passed and the light emittingelement OLED is placed in the light emitting state when the OLED is tobe driven. According to the present example, before the brightnessinformation of the next scanning line cycle (frame) is newly writtenafter writing the brightness information into the pixels in units of thescanning lines, the display device can extinguish the light emittingelements contained in the pixels in units of the scanning linestogether. This means that the time from the lighting to theextinguishing of the light emitting elements after the writing of thebrightness information can be adjusted. Namely, it means that the ratio(duty) of the light emitting time in one scanning line cycle can beadjusted. The adjustment of the light emitting time (duty) correspondsto the adjustment of the drive current supplied to each light emittingelement. Accordingly, it is possible to adjust the display brightnessconveniently and freely by adjusting the duty. A further important pointresides in that the drive current can be equivalently made large byadequately setting the duty. For example, when the duty is set at 1/10,even if the drive current is increased to 10 times, an equivalentbrightness is obtained. If the drive current is made 10 times large,also the signal current corresponding to this can be made 10 timeslarger, and therefore it is not necessary to handle a weak currentlevel.

[0080]FIG. 12 is another modification of the pixel circuit shown in FIG.9. In FIG. 12, a TFT6 is inserted between the drain of the TFT1 and thecathode of the light emitting element OLED, a TFT7 is connected betweenthe gate and the drain of the TFT6, and the gate thereof is controlledby the second scanning line SCAN-B. An auxiliary capacity C2 isconnected between the source of the TFT7 and the GND potential. Thedriving method of this circuit is basically the same as the case of thepixel circuit of FIG. 9, but will be explained below. Note that, thedrive waveform is similar to that of the case of FIG. 10. First, at thetime of writing, when the first scanning line SCAN-A and the secondscanning line SCAN-B are brought to the selected state in the statewhere the anode line A arranged in units of the scanning lines isbrought to the low level (for example GND or negative potential) and thecurrent is prevented from flowing through the OLED, the signal currentIw flows through the TFT1 and TFT6. Since the gates and the sources areshort-circuited by the TFT4 and TFT7, the two TFTs operate in thesaturated region. Next, the first scanning line SCAN-A and secondscanning line SCAN-B are brought to the unselected state. By this, theVgs previously created in the TFT1 and the TFT6 are held by thecapacitor C and the auxiliary capacitor C2. Next, by bringing the firstscanning line SCAN-A to the off state, the pixel circuit and the dataline DATA are electrically cut off, therefore the writing to anotherpixel can be carried out via the data line DATA after that. Then, theanode line A is set at a high level. Since the Vgs of the TFT1 is heldby the capacitor C, if the TFT1 operates in the saturated region, thecurrent flowing through the TFT1 coincides with Iw of equation (3). Thisbecomes the current Idrv flowing through the light emitting elementOLED. That is, the signal current Iw coincides with the drive currentIdrv of the light emitting element OLED.

[0081] Here, an explanation will be made of the function of the TFT6. Inthe pixel circuit of FIG. 9, as mentioned before, both of the signalcurrent Iw and the drive current of the light emitting element OLED aredetermined by the TFT1, therefore Iw=Idrv stood by equation (3) andequation (4). Note, this is true when assuming a case where the currentIds flowing through the TFT1 is given by equation (1) in the saturatedregion, that is, Ids does not depend on the voltage Vds between thedrain and the source. Nevertheless, in an actual transistor, even if Vgsis constant, the larger Vds, the larger Ids in a certain case. This isdue to the so-called short channel effect where a pinchoff point in thevicinity of the drain moves to the source by an increase of the Vds, andan effective channel length is reduced, or a so-called back gate effectwhere the potential of the drain exerts an influence upon the channelpotential, and the conduction rate of the channel changes, and so on. Inthis case, the current Ids flowing through the transistor becomes forexample as in the following equation.

Ids=μ·Cox·W/L/2(Vgs−Vth)^(2*()1+λ·Vds)  (9)

[0082] Accordingly, Ids will depend on Vds. Here, X is a positiveconstant. In this case, in the circuit of FIG. 9, Iw does not coincidewith Idrv unless Vds is not identical between the time of the writingand the time of the drive.

[0083] As opposed to this, the operation of the circuit of FIG. 12 willbe considered. When paying attention to the operation of the TFT6 ofFIG. 12, the drain potential thereof is not generally identical betweenthe time of the writing and the time of the drive. For example, wherethe drain potential at the time of the drive is higher, the Vds of theTFT6 becomes larger. When inserting this in equation (9), even if Vgs isconstant between the time of the writing and the time of the drive, Idsis increased at the time of the drive. In other words, Idrv becomesbigger than Iw, and the two do not coincide. However, the Idrv flowsthrough the TFT1, therefore, in that case, the voltage drop at the TFT1becomes large and the drain potential thereof (source potential of theTFT6) rises. As a result, Vgs of the TFT6 becomes small. This acts in adirection reducing the Idrv. As a result, the drain potential of theTFT1 (source potential of the TFT6) cannot largely fluctuate. Whenpaying attention to the TFT1, it is seen that Ids does not largelychange between the time of the writing and the time of the drive.Namely, Iw and Idrv will coincide with a remarkably high precision. Inorder to perform this operation better, it is good if the dependency ofIds with respect to Vds is made small in both of the TFT1 and TFT6,therefore desirably both transistors are operated in the saturatedregion. At the time of writing, the gate and the drain areshort-circuited in both of the TFT1 and TFT6. Therefore, regardless ofthe brightness data written, the two operate in the saturated region. Inorder to operate them also at the drive, a sufficient positive potentialmay be given to the anode line A so that the TFT6 still operates in thesaturated region even if the voltage drop at the light emitting elementOLED is considered. By this drive, the current Idrv flowing through thelight emitting element OLED more correctly coincides with the Iw thanthe embodiment of FIG. 9 without being affected by variations in thecharacteristics of the TFT. As described above, the drive part of thepresent example has TFT6, TFT7, and C2 as potential fixing means forfixing the potential of the drain with reference to the source of theconversion use thin film transistor TFT1 for stabilizing the currentlevel of the drive current flowing to the light emitting element OLEDthrough the conversion use thin film transistor TFT1.

[0084]FIG. 13 is another embodiment of the pixel circuit according tothe present invention. The characterizing feature of this pixel circuitresides in that, in the same way as FIG. 9, FIG. 11, and FIG. 12, thetransistor TFT1 per se with the signal current Iw flowing therethroughcontrols the current Idrv flowing through the light emitting elementOLED, but in FIG. 13, the light emitting element OLED is connected tothe source side of the TFT1. Namely, the drive part of the present pixelcircuit includes the thin film transistor TFT1 provided with the gate,drain, and the source and passes the drive current passing between thedrain and the source to the light emitting element OLED in accordancewith the level of the voltage applied to the gate. The light emittingelement OLED is a two-terminal type having an anode and a cathode, andthe anode is connected to the source. On the other hand, the drive partof the pixel circuit shown in FIG. 9 includes the thin film transistorprovided with the gate, drain, and the source and passes the drivecurrent passing between the drain and the source to the light emittingelement in accordance with the level of the voltage applied to the gate.The light emitting element is the two-terminal type having an anode anda cathode, and the cathode is connected to the drain.

[0085] The pixel circuit of the present example comprises, other thanthe TFT1, a transistor TFT3 for connecting or cutting off the pixelcircuit and the data line DATA by the control of the first scanning lineSCAN-A, a transistor TFT4 for short-circuiting the gate and the drain ofthe TFT1 during the writing period by the control of the second scanningline SCAN-B, a capacitor C for holding the gate potential of the TFT1even after the end of the writing, a P-channel transistor TFT5 insertedbetween the drain of the TFT1 and the power supply potential Vdd, andthe light emitting element OLED. In FIG. 13, one terminal of thecapacitor C is connected to the GND, and the Vgs of the TFT1 is held atschematically the same value between the time of the writing and thetime of the drive. Note that, the gate of the TFT5 is controlled by thedrive line drv. The object of the insertion of the TFT5 is to bring theTFT5 into the off state by setting the drive line drv at the high levelat the time of writing data and pass all of the signal current Iwthrough the TFT1. After the writing is ended, the drv is brought to thelow level, the TFT5 is brought to the on state, and the drive currentIdrv is passed through the light emitting element OLED. In this way, thedriving method is similar to that of the circuit of FIG. 11.

[0086]FIG. 14 is a modification of the pixel circuit shown in FIG. 13.In FIG. 13 and FIG. 14, the difference resides in that one terminal ofthe capacitor C is connected to the GND in FIG. 13, but is connected tothe source of the TFT1 in FIG. 14, but in both cases, there is nofunctional difference in the point that the Vgs of the TFT1 is held atschematically the same value between the time of the writing and thetime of the drive.

[0087]FIG. 15 is a more developed example of the pixel circuit shown inFIG. 5. The present pixel circuit includes an adjusting means fordownwardly adjusting the voltage level held by the converting part andsupplying the same to the drive part to tighten the black level of thebrightness of each pixel. Concretely, the drive part includes a thinfilm transistor TFT2 having a gate, drain, and source and an adjustingmeans provided with a constant voltage source E for raising the bottomof the voltage between the gate and the source of the thin filmtransistor TFT2 and downwardly adjusting the level of the voltageapplied to the gate. Namely, it tightens the black level by connectingthe source of the TFT2 to the potential E slightly higher than thesource potential of the TFT1.

[0088]FIG. 16 is a modification of the pixel circuit shown in FIG. 15.In the present example, the adjusting procedure is comprised by anadditional capacitor C2 connected to the gate of the thin filmtransistor TFT2 and the second scanning line SCAN-B and downwardlyadjusts the voltage level to be held at the capacitor C for applying thesame to the gate of the thin film transistor TFT2. Namely, whenswitching the second scanning line SCAN-B to the low level and bringingit to the unselected state, the gate potential of the TFT2 can beslightly lowered by the function of the capacitor C2. As describedabove, in the present display device, the scanning line SCAN-A forselecting the pixel and the data line DATA giving the brightnessinformation for driving the pixel are arranged in the matrix state. Eachpixel includes the light emitting element OLED having the brightnesschanging according to the amount of the supplied current, the writingmeans (TFT1, TFT3, C) controlled by the scanning line SCAN-A and writingthe brightness information given from the data line DATA to the pixel,and the driving means (TFT2) for controlling the amount of the currentsupplied to the light emitting element OLED in accordance with thewritten brightness information. The brightness information is writteninto each pixel by applying the electric signal Iw in accordance withthe brightness information to the data line DATA in the state where thescanning line SCAN A is selected. The brightness information written ineach pixel is held at each pixel even after the scanning line SCAN-Abecomes unselected. The light emitting element OLED of each pixelincludes the adjusting means (C2) capable of maintaining the lightingwith the brightness in accordance with the held brightness information,downwardly adjusting the brightness information written by the writingmeans (TFT1, TFT3, C), and supplying the same to the drive means (TFT2)and can tighten the black level of the brightness of each pixel.

[0089]FIG. 17 is a modification of the pixel circuit shown in FIG. 15.In the present example, the adjusting procedure downwardly adjusts thelevel of the voltage to be applied to the gate of the TFT2 by adjustingthe potential of one end of the capacitor C when holding the voltagelevel converted by the TFT1 at the capacitor C. Namely, by controllingthe source potential control line S connected to one end of thecapacitor C, the black level is tightened. This is because the gatepotential of the TFT2 is slightly lowered by the function of thecapacitor C when setting the potential control line S at a lowerpotential than that at the writing. The potential control line S isprovided in units of the scanning lines and, controlled. The potentialcontrol line S is brought to an “H” level during the writing and broughtto an “L” level after the end of the writing. When defining an amplitudeas ΔVs and defining the capacity existing at the gate of the TFT2 (gatecapacity, other parasitic capacity) as Cp, the gate potential of theTFT2 is lowered by exactly ΔVg=ΔVs*C/(C+Cp), and Vgs becomes small. Theabsolute values of the H and L potentials can be freely set.

[0090]FIG. 18 is another embodiment of the pixel circuit according tothe present invention. In the circuit of the present example, the fetchuse thin film transistor TFT3 and the switch use thin film transistorTFT4 are configured as the identical conductivity type (PMOS in FIG.18). Then, in the present example, as shown in FIG. 18, it is alsopossible to connect their gates to the common scanning line SCAN in thewrite operation and control them by the common signal. In the devicedisplay in this case, the scanning line drive circuit B23 in the displaydevice shown in FIG. 7 is unnecessary.

[0091]FIG. 19 is a modification of the pixel circuit shown in FIG. 18.In the present example, in the same way as the circuits shown in FIG. 5,FIG. 8, FIG. 9, and FIG. 11 to FIG. 17, the gates of the fetch use thinfilm transistor TFT3 and switch use thin film transistor TFT4 configuredby the same conductivity type P-channel TFT are connected to differentscanning lines, that is, the first scanning line SCAN-A and the secondscanning line SCAN-B, and separately controlled. The reason why they areseparately controlled in this way is that, if the TFT3 and the TFT4 arecontrolled by the common signal as in the example of FIG. 18, thefollowing inconvenience sometimes occurs.

[0092] When the write operation with respect to the pixel on a certainscanning line is terminated, at the rise of the level of the scanningline SCAN in the example of FIG. 18, the impedance of the TFT3 isinevitably increase and finally actually becomes infinitely large, thatis, the off state. Accordingly, in this step, the potential of the dataline DATA gradually rises, but at a point of time when it rises to acertain degree, the current source for driving the data line DATA losesthe constant current property, and the current value is decreased.

[0093] As a concrete example, an example where the data line DATA isdriven by a PNP transistor BIP1 as in FIG. 18 is considered. When thecurrent flowing through the base is the constant value Ib and a currentamplification rate of a transistor IBIP1 is β, if a certain degree ofthe voltage (for example 1V) is applied between the collector and theemitter of the transistor BIPI, the transistor BIP1 operates assubstantially a constant current source, and a current of a magnitude ofIw=βIb is supplied to the data line DATA. However, at the end of thewrite operation, when the impedance of the TFT3 rises, the potential ofthe data line rises, and when the transistor BIP1 enters into thesaturated region, it loses the constant current property, and the drivecurrent is decreased from βIb. At this time, if the TFT4 is in the onstate, this decreased value of the current flows through the TFT1, andthe intended value of the current will not be correctly written.

[0094] Accordingly, more desirably the TFT3 and the TFT4 are controlledby the different signal lines, that is, the first scanning line SCAN-Aand the second scanning line SCAN-B, and the TFT4 is brought to the offstate preceding the TFT3 at the end of the write operation. In the pixelcircuit according to the present invention, the TFT3 and the TFT4 do nothave to be the same conductivity type as in the examples mentionedbefore. The pixel circuit may be configured so that the TFT3 and theTFT4 are the identical or different conductivity types, their gates arecontrolled by the different scanning lines such as the SCAN-A and theSCAN-B, and the TFT4 is brought to the off state preceding the TFT3 atthe end of the write operation. This is true also for the examplesexplained before by referring to the drawings.

[0095] Further, when the TFT3 and the TFT4 are controlled by thedifferent scanning lines SCAN-A and SCAN-B, after the end of the writeoperation, the TFT4 is brought to the on state by the operation of thesecond scanning line SCAN-B, and the pixels can be extinguished in unitsof the scanning lines. This is because the gate and the drain of theTFT1 and the gate of the TFT2 are connected, so the gate voltage of theTFT2 becomes the threshold value of the TFT1 (this is almost equal tothe threshold value of the TFT2), and both of the TFT1 and TFT2 becomethe off state. In the waveform of the second SCAN-B, as shown in FIG.20(B), it is also possible to give a pulse-like extinguishing signal, orit is also possible to give a continuous extinguishing signal as SCAN-Bshown in FIG. 20(C).

[0096] In this way, by changing the timing of the extinguishing signal,it is possible to conveniently and freely change the brightness of thedisplay device. If the second scanning line SCAN-B is divided for eachof the colors of R, G, and B and they are separately controlled, thecolor balance can also be easily adjusted.

[0097] Further, when it is desired to obtain the same time averagebrightness, by reducing the ratio of the light emission period (duty),the drive current of the light emitting element OLED can be made large.This means that a write current larger by that amount is handled.Therefore, the realization of the write drive circuit to the data lineDATA becomes easy, and also a write required time can be shortened.Further, by reducing the light emission duty to about 50% or less, themoving picture image quality is improved.

[0098] Further, in the same way as the circuits shown in FIG. 5, FIG. 8,FIG. 9, and FIG. 11 to FIG. 18, in the circuit of FIG. 19, the fetch usethin film transistor TFT3 and the conversion use thin film transistorTFT1 are configured as different conductivity types. For example, wherethe conversion use thin film transistor TFT1 is the N-channel type, thefetch use thin film transistor TFT3 is configured as the P-channel type.This is for the following reason.

[0099] Namely, desirably the fluctuation of the potential of the dataline is as small as possible when configuring the constant current drivecircuit for driving the data line. This is because, as mentioned before,if the amount of fluctuation of the data line potential is wide, theconstant current property is easily lost in the data line drive circuit.In addition, the amplitude of the scanning line SCAN-A for reliablyturning on or off the TFT3 becomes large. This is disadvantageous in thepoint of the consumed power.

[0100] Accordingly, desirably the voltage drop of the route reaching theground potential from the data line via the TFT3 and the TFT1 is small.Therefore, in contrast to the example of FIG. 19 wherein the TFT1 is anNMOS, the TFT3 is configured by a PMOS, and the voltage drop at the TFT3is suppressed small. Namely, the voltage drop at the TFT3 becomes themaximum when the value of the write current Iw is the maximum.Therefore, in order to suppress the amplitude of the data line small,the voltage drop at the TFT3 when the write current Iw is the maximumshould be made small. In the example of FIG. 19, when the write currentIw is large, the potential of the data line rises in accordance withthat, but the absolute value of the voltage between the gate and thesource of the TFT3 is increased along with that and the impedance of theTFT3 is lowered. Contrary to this, if the TFT3 is an NMOS, the largerthe write current Iw, the smaller the voltage between the gate and thesource, the greater the impedance of the TFT3, and the more easily arise of the data line potential is induced. Similarly, when the TFT1 isconfigured by a PMOS, the TFT3 is preferably configured by a NMOS.

[0101] Note that a practical configuration can be realized whether theconductivity type of the TFT4 is the same as or different from the TFT3,but if the TFT4 is given the same conductivity type as that of the TFT3,the first scanning line SCAN-A and the second scanning line SCAN-B areeasily driven by the common potential, so this is more desirable.

[0102]FIG. 21 is a modification of the pixel circuit shown in FIG. 19.The pixel circuit according to the present example is similar to thepixel circuit shown in FIG. 19 in terms of the equivalent circuit, butit is different from the circuit of FIG. 19 in the point that the ratioW/L between the channel width (W) and the channel length (L) of theconversion use thin film transistor TFT1 is set larger than the W/L ofthe drive use thin film transistor TFT2. The reason for setting the W/Lof the TFT1 larger than the W/L of the TFT2 in this way is for reliablyending the write operation. An explanation will be made of this below bygiving specific figures.

[0103] As practical numbers, when the maximum brightness is 200 cd/M²,the size of the light emitting surface per pixel is 100 μm×100 μm=1e−8m², and the light emission efficiency is 2 cd/A, the drive current ofthe light emitting element OLED at the maximum brightness becomes200×1e−8/2=1 μA. When it is intended to control 64 tones, the currentvalue corresponding to the minimum tone becomes about 1 μA/64=16 nA. Itis extremely difficult to correctly supply such a small current value.Further, the TFT1 operates in the state of high impedance, therefore along time is taken for stabilization of the state of the circuit due toan influence of a parasitic capacitance of the data line DATA, etc. Thewrite operation sometimes cannot be terminated within the predeterminedscanning line cycle.

[0104] As shown in FIG. 21, if W/L of TFT1=100/10 and W/L of TFT2=5/20,the ratio of W/L becomes 40, the write current to be supplied to thedata line DATA for obtaining the OLED drive current of 16 nA becomes 16nA×40=640 nA, which is a practical number, so the write operation can bereliably terminated. When the TFT1 and the TFT2 comprise a plurality oftransistors, the above calculation naturally should be carried out byconsidering an effective W/L.

[0105]FIG. 22 is a more developed example of the circuit shown in FIG.19. In the present pixel circuit, a leak element LEK1 is connectedbetween each data line DATA and the predetermined potential to try tospeed up of black writing.

[0106] In the current write type pixel circuit, a case of writing“black” corresponds to a case where the write current is zero. At thistime, when assuming that a “white” level, that is a relatively largecurrent, is written into the data line in the scanning line cycleimmediately before that and as a result the data line potential hasbecome a relatively high level, a long time is necessary for writing“black” immediately after that. The writing of “black” means thatinitial charges stored in a capacitor Cd etc. of the data line aredischarged, but when the data line potential is lowered and becomes inthe vicinity of the threshold value of the TFT1, the impedance of theTFT1 becomes high, and as indicated by a characteristic curve <1> inFIG. 23 showing the characteristic of the current flowing through theTFT1, theoretically the “black” writing is permanently not terminated.In actuality, the write operation is carried out in a finite time,therefore this appears as a so-called “black float” phenomenon where the“black” level is not completely achieved. This lowers the contrast ofthe image.

[0107] Therefore, in the circuit of FIG. 22, the leak element LEK1,concretely the NMOS transistor, is connected between the data line DATAand the ground potential GND, and a constant bias is given as Vg. Bythis, as indicated by a characteristic curve <2> in FIG. 22, the “black”writing is reliably terminated. As the leak element LEK1, also a simpleresistor may be used, but in that case, when the data line potentialrises at the “white” writing, the current flowing through the resistoris increased in proportion to that, and this induces the lowering of thecurrent flowing through the TFT1 and the degradation of the powerconsumption. Contrary to this, if an NMOS is operated in the saturatedregion, a constant current operation is achieved, therefore such a badinfluence can be suppressed small. Note that, it is also possible tocomprise the leak element by an TFT or comprise the same by an externalpart separately from the TFT process.

[0108]FIG. 24 is a more developed example of the circuit shown in FIG.19. In the present pixel circuit, an initial value setting element PRC1is connected between each data line DATA and the predeterminedpotential, and the initial value of the data line is set preceding thewrite operation by the operation of that element to speed up the writeoperation.

[0109] In the current write type pixel, there is a case where a longtime is required when writing gray near black. In FIG. 25, a case wherethe potential of the data line at the start of the write operation is 0Vis shown. This can occur in the case where “black” is written in thescanning line cycle immediately before that, the case where thethreshold value Vth1 of the TFT1 of the written pixel is low, i.e. about0V, or similarly the case of the black writing, and the case where theleak element for the countermeasure of black float is provided.

[0110] In the conventional circuit, gray near “black”, that is, a verysmall current value, is written from 0V as the initial value, thereforea long time is taken for reaching the balance potential VBLA. Forexample, as indicated by the characteristic curve <1> in FIG. 25, it canbe also considered that the threshold value of the TFT1 is not reachedwithin the predetermined write time, but in this case, the TFT2 alsobecomes the off state, the gray cannot be correctly written, and thedisplay image exhibits a so-called black crushed state.

[0111] In the circuit of FIG. 24, a PMOS transistor is connected betweenthe data line and the power supply potential Vdd as the initial valuesetting (precharging) element PRC1, and the first pulse is given at thefirst writing cycle as the gate potential Vg. By this pulse application,as indicated by the characteristic curve <2> in FIG. 25, the data linepotential rises to the threshold value Vth1 of the TFT1 or more andconverges thereafter at relatively a high speed toward the balancepotential VBLA determined by a balance between the write current Iw andthe operation of the TFT inside the pixel, so correct writing of thebrightness data at a high speed becomes possible. Note that, it is alsopossible to configure the precharge use element by a TFT or configurethe same by an external part separately from the TFT process.

[0112]FIG. 26 is another embodiment of the pixel circuit according tothe present invention. In this circuit, unlike the circuits of theexamples mentioned before, the conductivity types of the TFT1 and theTFT2 are achieved by the P-channel type (PMOS). Along with this, for theabove reason, the TFT3 is configured as the N-channel type (NMOS) as aconductivity type different from that of the TFT1. The TFT4 isconfigured as the N-channel type (NMOS) as the identical conductivitytype to that of the TFT3 in consideration with the controllability.

[0113] In the circuit of FIG. 26, the two transistors TFT1 and TFT2operate by equal gate-source voltages at the time of driving the lightemitting element OLED, but the drain-source voltages are not alwaysequal. In order to achieve a correct proportion between the writecurrent Iw and the drive current of the light emitting element OLED,desirably the TFT2 is operated in the saturated region as previouslymentioned. On the other hand, in the case of an NMOS, generally an LDD(lightly doped drain) structure is employed in order to improve thewithstand voltage. This is because, in this case, the drain current iseasily influenced by the drain-source voltage in the saturated region.In other words, the constant current property tends to be inferior to anPMOS due to a serial resistance component by the LDD.

[0114] Accordingly, preferably the conversion use thin film transistorTFT1 and the drive use thin film transistor TFT2 are configured byPMOSS.

[0115] The operation of this circuit is basically similar to that of thecircuit of FIG. 5 etc. except for the point that the polarities of theelements become reverse.

[0116]FIG. 27 shows another embodiment of the pixel circuit according tothe present invention. Unlike the circuits of the examples mentionedabove, this circuit is configured so that, in place of connecting theswitch use thin film transistor TFT4 between the drain and the gate ofthe conversion use thin film transistor TFT1, the drain and the gate ofthe TFT1 are directly connected, and the TFT4 is connected between aconnection point of them and the connection point between the gate ofthe TFT2 and the capacitor.

[0117] Also in this circuit of FIG. 27, basically the operation the sameway as that in the circuit of FIG. 5 etc. is possible. Further, also inthis circuit, the TFT3 and the TFT4 may be identical or differentconductivity types, the gates of them are controlled by differentscanning lines such as the first scanning line SCAN-A and the secondscanning line SCAN-B, and the TFT4 brought to the off state precedingthe TFT3 at the end of the write operation. Further, as explained inrelation to FIG. 21, in order to reliably terminate the write operationin the predetermined scanning line cycle, desirably the size (W/L) ofthe TFT1 is set larger than the size of the TFT2.

Industrial Applicability

[0118] As described above, by the current drive circuit according to thepresent invention and the display device using the same, it is possibleto pass a drive current Idrv correctly proportional (or corresponding)to the signal current Iw from a data line through a current drive typelight emitting element (organic EL element or the like) without beingaffected by variations in the characteristics of the active element (TFTetc.) By arranging a large number of pixel circuits including suchcurrent drive circuits in a matrix, each pixel can be made to correctlyemit light with the intended brightness. Therefore it is possible toprovide a high quality active matrix type display device.

List of References

[0119] OLED . . . light emitting element

[0120] TFT1 . . . conversion use thin film transistor

[0121] TFT2 . . . drive use thin film transistor

[0122] TFT3 . . . fetch use thin film transistor

[0123] TFT4 . . . switch use thin film transistor

[0124] C . . . holding capacitor

[0125] CS . . . current source

[0126] SCAN-A . . . scanning line

[0127] SCAN-B . . . scanning line

[0128] DATA . . . data line

[0129]21 . . . scanning line drive circuit

[0130]22 . . . data line drive circuit

[0131]25 . . . pixel

1. A current drive circuit for supplying a drive current to a drivenobject, including: a control line, a signal line to which a signalcurrent having a current level in accordance with information issupplied, a receiving part for fetching the signal current from thesignal line when the control line is selected, a converting part forconverting a current level of the fetched signal current to a voltagelevel and holding the same, and a drive part for converting the heldvoltage signal to a current signal and outputting the drive current. 2.A drive current circuit as set forth in claim 1, wherein the convertingpart includes a conversion use transistor provided with a controlterminal, a first terminal, and a second terminal and a capacitorconnected to the control terminal.
 3. A current drive circuit as setforth in claim 2, wherein the converting part includes a switch usetransistor inserted between the first terminal and control terminal ofthe conversion use transistor; the switch use transistor becomesconductive when converting the current level of the signal current tothe voltage level and electrically connects the first terminal and thecontrol terminal of the conversion use transistor to create the voltagelevel with reference to the second terminal at the gate; and the switchuse transistor is cut off when the capacitor holds the voltage level andseparates the control terminal of the conversion use transistor and thecapacitor connected to this from the first terminal.
 4. A current drivecircuit as set forth in claim 1, wherein the receiving part includes afetch use insulating gate type field effect transistor having a controlterminal, a first terminal, and a second terminal, the first terminalconnected to a first terminal of the conversion use transistor thesecond terminal connected to the signal line, and the control terminalconnected to the control line and the converting part includes a switchuse transistor inserted between the first terminal and control terminalof the conversion use transistor.
 5. A current drive circuit as setforth in claim 4, wherein the control terminal of the fetch usetransistor and the control terminal of the switch use transistor areconnected to different control lines.
 6. A current drive circuit as setforth in claim 4, wherein a conductivity type of the conversion usetransistor and a conductivity type of the fetch use transistor aredifferent.
 7. A current drive circuit as set forth in claim 2, whereinthe drive part includes a drive use transistor provided with a controlterminal, a first terminal, and a second terminal and the drive usetransistor receives a voltage level held at the capacitor at its controlterminal and passes a drive current having a current level in accordancewith the same.
 8. A current drive circuit as set forth in claim 7,wherein the control terminal of the conversion use transistor and thecontrol terminal of the drive use transistor are directly connected toconfigure a current mirror circuit and the current level of the signalcurrent and the current level of the drive current become proportional.9. A current drive circuit as set forth in claim 7, wherein the driveuse transistor is formed in the vicinity of the conversion usetransistor and has a equal threshold voltage as the conversion usetransistor.
 10. A current drive circuit as set forth in claim 7, whereinthe size of the conversion use transistor is set larger than the size ofthe drive use transistor.
 11. A current drive circuit as set forth inclaim 9, wherein the drive use transistor operates in the saturatedregion and passes a drive current corresponding to the differencebetween the voltage level applied to the gate and the threshold voltage.12. A current drive circuit as set forth in claim 9, wherein the driveuse transistor operates in the linear region.
 13. A current drivecircuit as set forth in claim 10, wherein the drive use transistoroperates in the linear region. 14-17. (canceled)
 18. A current drivecircuit as set forth in claim 1, wherein the receiving part, convertingpart, and drive part configures a current circuit comprised of aplurality of transistors and at least one transistor has a double-gatestructure for suppressing current leakage in the current circuit.
 19. Acurrent drive circuit as set forth in claim 1, wherein a leak element isconnected between said data line and a predetermined potential.
 20. Acurrent drive circuit as set forth in claim 1, wherein an initial valuesetting element for setting the data to an initial value is connectedbetween said data line and a predetermined potential.
 21. A currentdrive circuit as set forth in claim 7, wherein said drive use insulatinggate type field effect transistor is a P-channel type.
 22. A currentdrive circuit for supplying a drive current to a driven object,including: at least one control line, a signal line to which a signalcurrent having a current level in accordance with information issupplied, a conversion use insulating gate type field effect transistorwith a source connected to a reference potential, a fetch use insulatinggate type field effect transistor connected between a drain of saidconversion use insulating gate type field effect transistor and saidsignal line and having a gate connected to a said control line, a driveuse insulating gate type field effect transistor connected between thereference potential and said driven object, a capacitor having a firstelectrode connected in common to a gate of said conversion useinsulating gate type field effect transistor and a gate of said driveuse insulating gate type field effect transistor and having a secondelectrode connected to the reference potential, and a switch useinsulating gate type field effect transistor connected between a gateand drain of said conversion use insulating gate type field effecttransistor and having a gate connected to said control line.
 23. Acurrent drive circuit for supplying a drive current to a driven object,including: at least one control line, a signal line to which a signalcurrent having a current level in accordance with information issupplied, a conversion use insulating gate type field effect transistorwith a source connected to a reference potential, a fetch use insulatinggate type field effect transistor connected between a drain of saidconversion use insulating gate type field effect transistor and saidsignal line and having a gate connected to said control line, a driveuse insulating gate type field effect transistor connected between thereference potential and said driven object, a capacitor having a firstelectrode connected to a gate of said drive use insulating gate typefield effect transistor and having a second electrode connected to areference potential, and a switch use insulating gate type field effecttransistor connected between a gate of said conversion use insulatinggate type field effect transistor and a connecting point of a gate ofsaid drive use insulating gate type field effect transistor and a firstelectrode of said capacitor and having a gate connected to said controlline.
 24. A current drive circuit as set forth in claim 23, wherein acontrol terminal of said fetch use insulating gate type field effecttransistor and a control terminal of said switch use insulating gatetype field effect transistor are connected to different control lines.25. A current drive circuit as set forth in claim 23, wherein a size ofsaid conversion use transistor is set larger than a size of said driveuse transistor.
 26. A display device, comprising: a scanning line, adata line to which a signal in accordance with brightness information issupplied, and a pixel comprising a display element formed at anintersecting portion of said data line and said scanning line, saidpixel comprising a receiving part for fetching the signal supplied tothe data line when the scanning line is selected, a converting andholding part for converting and holding the fetched signal, and a drivepart for converting the held signal and supplying it to said displayelement.
 27. A display device as set forth in claim 26, wherein saidfetched signal is a current, the signal held at said converting andholding part is a voltage, and the signal supplied to said displayelement is a current.
 28. A display device as set forth in claim 26,wherein said converting and holding part comprises a first transistorprovided with a control terminal and one end of a capacitor connected tosaid control terminal.
 29. A display device as set forth in claim 28,wherein said converting and holding part comprises a second transistorconnected between the first terminal of said first transistor and saidcontrol terminal.
 30. A display device as set forth in claim 29, whereinsaid second transistor becomes conductive in state when said signalsupplied to the data line is fetched by said receiving part and becomesnonconductive in state after the signal is supplied to said convertingand holding part.
 31. A display device as set forth in claim 29, whereinsaid receiving part comprises a third transistor having a first terminalconnected to the first terminal of the first transistor and a secondterminal connected to said data line and the control terminal of saidsecond transistor and the control terminal of said third transistor areconnected to different scanning lines.
 32. (canceled)
 33. A displaydevice as set forth in claim 28, wherein said drive part comprises athird transistor having a control terminal connected to the controlterminal of said first transistor.
 34. A display device as set forth inclaim 29, wherein said drive part comprises a third transistor having acontrol terminal connected to the control terminal of said firsttransistor and wherein said first, second, and third transistorsconfigure a current mirror circuit. 35-37. (canceled)
 38. A displaydevice as set forth in claim 26, wherein said drive part and saidconverting and holding part are configured by a plurality oftransistors.
 39. A display device as set forth in claim 26, wherein saidconverting and holding part comprises a plurality of transistorsprovided with control terminals and a plurality of capacitors connectedto the control terminals.
 40. A display device as set forth in claim 33,wherein said display element is connected to the first terminal of saidthird transistor and a constant voltage source is connected to thesecond terminal of said third transistor.
 41. A display device as setforth in claim 34, wherein the control terminal of said secondtransistor is connected to said capacitor.
 42. (canceled)
 43. A displaydevice as set forth in claim 26, wherein said display element has atleast one transparent electrode and has a layer including an organicsubstance sandwiched between said electrodes.
 44. A display device asset forth in claim 26, wherein a leak element is connected between saiddata line and a predetermined potential.
 45. A display device as setforth in claim 26, wherein an initial value setting element for settingsaid data to an initial value before said scanning line is selected isconnected between said data line and a predetermined potential.
 46. Adisplay device comprising: a scanning line, a data line to which acurrent signal in accordance with brightness information is supplied,and a pixel comprising an organic layer formed at an intersectingportion of said data line and said scanning line, said pixel comprisinga receiving part for fetching the current signal supplied to the dataline when the scanning line is selected, a converting and holding partfor converting the fetched current signal to a voltage and holding thesame, and a drive part for converting the held voltage signal andsupplying a current to said display element.
 47. A display device as setforth in claim 46, wherein said brightness information is a voltage andwherein the voltage is converted to a current and supplied to the dataline.
 48. A display device as set forth in claim 46, wherein saidconverting and holding part comprises a first transistor provided with acontrol terminal and one end of a capacitor connected to said controlterminal.
 49. A display device as set forth in claim 48, wherein saidconverting and holding part comprises a second transistor connectedbetween the first terminal of said first transistor and said controlterminal.
 50. A display device as set forth in claim 49, wherein saidsecond transistor becomes conductive in state when said signal suppliedto the data line is fetched by said receiving part and becomesnonconductive in state after the signal is supplied to said convertingand holding part.
 51. A display device as set forth in claim 49, whereinsaid receiving part comprises a third transistor having a first terminalconnected to the first terminal of said first transistor and a secondterminal connected to said data line and the control terminal of saidsecond transistor and the control terminal of said third transistor areconnected to different scanning lines.
 52. (canceled)
 53. A displaydevice as set forth in claim 48, wherein said drive part comprises athird transistor having a control terminal connected to the controlterminal of said first transistor.
 54. A display device as set forth inclaim 49, wherein said drive part comprises a third transistor having acontrol terminal connected to the control terminal of said firsttransistor and wherein said first, second, and third transistorsconfigure a current mirror circuit. 55-57. (canceled)
 58. A displaydevice as set forth in claim 46, wherein said drive part and saidconverting and holding part are configured by a plurality oftransistors. 59-60. (canceled)
 61. A display device as set forth inclaim 54, wherein the control terminal of said second transistor isconnected to said capacitor.
 62. (canceled)
 63. A display device as setforth in claim 46, wherein said display element has at least onetransparent electrode and has a layer including an organic substancesandwiched between said electrodes.
 64. A display device as set forth inclaim 46, wherein a leak element is connected between said data line anda predetermined potential.
 65. A display device as set forth in claim46, wherein an initial value setting element for setting said data to aninitial value before said scanning line is selected is connected betweensaid data line and a predetermined potential.
 66. A display devicecomprising a scanning line drive circuit for successively selectingscanning lines, a data line drive circuit including a current source forgenerating a signal current having a current level in accordance withbrightness information and successively supplying the same to datalines, and a plurality of pixels arranged at intersecting portions ofthe scanning lines and the data lines and including current driven typelight emitting elements emitting light by receiving the supply of thedrive current, wherein each pixel comprises a receiving part forfetching the signal current from a data line when the scanning line isselected, a converting part for converting a current level of thefetched signal current to a voltage level and holding the same, and adrive part for passing a drive current having a current level inaccordance with the held voltage level through the light emittingelement.
 67. A display device as set forth in claim 66, wherein theconverting part includes a conversion use insulating gate type fieldeffect transistor provided with a gate, a source, a drain, and a channeland a capacitor connected to the gate.
 68. A display device as set forthin claim 67, wherein the converting part includes a switch useinsulating gate type field effect transistor inserted between the drainand the gate of the conversion use insulating gate type field effecttransistor, the switch use insulating gate type field effect transistorbecomes conductive when converting the current level of the signalcurrent to the voltage level and electrically connects the drain and thegate of the conversion use insulating gate type field effect transistorto create the voltage level with the source as the reference at thegate, and the switch use insulating gate type field effect transistor iscut off and separates the gate of the conversion use insulating gatetype field effect transistor and the capacitor connected to this fromthe drain when the capacitor holds the voltage level.
 69. A displaydevice as set forth in claim 66, wherein: the receiving part includes afetch use insulating gate type field effect transistor inserted betweenthe drain of the conversion use insulating gate type field effecttransistor and the data line and the converting part includes a switchuse insulating late type field effect transistor inserted between thedrain and the gate of the conversion use insulating gate type fieldeffect transistor.
 70. A display device as set forth in claim 69,wherein the gate of the fetch use insulating gate type field effecttransistor and the gate of the switch use insulating gate type fieldeffect transistor are connected to different scanning lines.
 71. Adisplay device as set forth in claim 70, wherein the switch useinsulating gate-type field effect transistor becomes conductive whenconverting the current level of the signal current to the voltage leveland electrically connects the drain and the gate of the conversion useinsulating gate type field effect transistor to create the voltage levelwith the source as the reference at the gate, the switch use insulatinggate type field effect transistor is cut off and separates the gate ofthe conversion use insulating gate type field effect transistor and thecapacitor connected to this from the drain when the capacitor holds thevoltage level, and the switch use insulating gate type field effecttransistor becomes unselected and is cut off before the fetch useinsulating gate type field effect transistor becomes nonconductive. 72.A display device as set forth in claim 71, wherein the switch useinsulating gate type field effect transistor is made conductive after apredetermined time within one frame period after the switch useinsulating gate type field effect transistor and the fetch useinsulating gate type field effect transistor become nonconductive toextinguish in units of scanning lines.
 73. A display device as set forthin claim 71, wherein a scanning line to which the switch use insulatinggate type field effect transistor is connected is provided independentlyfor each of the three primary colors.
 74. A display device as set forthin claim 69, wherein a conductivity type of said switch use insulatinggate type field effect transistor and a conductivity type of said fetchuse insulating gate type transistor are different.
 75. A display deviceas set forth in claim 67, wherein said drive part includes a drive useinsulating gate type field effect transistor provided with a gate, adrain, a source, and a channel, and the drive use insulating gate typefield effect transistor receives the voltage level held at the capacitorat its gate and passes a drive current having a current level inaccordance with that through the light emitting element via the channel.76. A display device as set forth in claim 75, wherein the gate of theconversion use insulating gate type field effect transistor and the gateof the drive use insulating gate type field effect transistor aredirectly connected to configure a current mirror circuit and wherein thecurrent level of the signal current and the current level of the drivecurrent are proportional.
 77. A display device as set forth in claim 75,wherein the drive use insulating gate type field effect transistor isformed in the vicinity of the corresponding conversion use insulatinggate type field effect transistor inside the pixel and has an equivalentthreshold voltage to that of the conversion use insulating gate typefield effect transistor.
 78. A display device as set forth in claim 77,wherein the size of the conversion use insulating gate type field effecttransistor is set larger than the size of the drive use insulating gatetype field effect transistor.
 79. A display device as set forth in claim77, wherein the drive use insulating gate type field effect transistoroperates in the saturated region and passes a drive current inaccordance with a difference between the level of the voltage applied tothe gate thereof and the threshold voltage through the light emittingelement.
 80. A display device as set forth in claim 77, wherein thedrive use insulating gate type field effect transistors operates in thelinear region.
 81. A display device as set forth in claim 78, whereinthe drive use insulating gate type field effect transistors operates inthe linear region. 82-87. (canceled)
 88. A display device as set forthin claim 66, wherein the receiving part, the converting part, and thedrive part configure a current circuit combining a plurality ofinsulating gate type field effect transistors, and one or two or moreinsulating gate type field effect transistors have a double gatestructure for suppressing current leakage in the current circuit.
 89. Adisplay device as set forth in claim 66, wherein the drive part includesan insulating gate type field effect transistor provided with a gate,drain, and a source and passes the drive current passing between thedrain and the source to the light emitting element in accordance withthe level of the voltage applied to the gate, and the light emittingelement is a two terminal type having an anode and a cathode, where thecathode is connected to the drain.
 90. A display device as set forth inclaim 66, wherein the drive part includes an insulating gate type fieldeffect transistor provided with a gate, a drain, and a source and passesa drive current passing between the drain and the source to the lightemitting element in accordance with the level of the voltage applied tothe gate, and the light emitting element is a two terminal type havingan anode and a cathode, where the anode is connected to the source. 91.A display device as set forth in claim 66, further including anadjusting means for downwardly adjusting the voltage level held by theconverting part and supplying the same to the drive part to tighten theblack level of the brightness of each pixel.
 92. A display device as setforth in claim 66, wherein a leak element is connected between said dataline and a predetermined potential.
 93. A display device as set forth inclaim 66, wherein an initial value setting element for setting said datato an initial value before said scanning line is selected is connectedbetween said data line and a predetermined potential.
 94. A displaydevice as set forth in claim 93, wherein the drive part includes aninsulating gate type field effect transistor having a gate, a drain, anda source, and the adjusting means downwardly adjusts the level of thevoltage applied to the gate by raising the bottom of the voltage betweenthe gate and the source of the insulating gate type field effecttransistor.
 95. A display device as set forth in claim 93, wherein thedrive part includes an insulating gate type field effect transistorhaving a gate, a drain, and a source, the converting part is providedwith a capacitor connected to the gate of the thin film transistor andholding the voltage level, and the adjusting means comprises anadditional capacitor connected to that capacitor and downwardly adjuststhe level of the voltage to be applied to the gate of the insulatinggate type field effect transistor held at that capacitor.
 96. A displaydevice as set forth in claim 93, wherein the drive part includes aninsulating gate type field effect transistor having a gate, a drain, anda source, the converting part is provided with a capacitor connected tothe gate of the insulating gate type field effect transistor on its oneend and holding the voltage level, and the adjusting means adjusts thepotential of the other end of the capacitor when holding the voltagelevel converted by the converting part at that capacitor to downwardlyadjust the level of the voltage to be applied to the gate of theinsulating gate type field effect transistor.
 97. A display device asset forth in claim 66, wherein the light emitting element comprises anorganic electroluminescence element.
 98. A display device as set forthin claim 75, wherein the drive use insulating gate type field effecttransistor comprises a P-channel type.
 99. A display device comprising ascanning line drive circuit for successively selecting scanning lines, adata line drive circuit including a current source for generating asignal current having a current level in accordance with brightnessinformation and successively supplying the same to data lines, and aplurality of pixels arranged at intersecting portions of the scanninglines and the data lines and including current driven type lightemitting elements emitting light by receiving the supply of the drivecurrent, wherein each pixel comprises a conversion use insulating gatetype field effect transistor having a source connected to a referencepotential, a fetch use insulating gate type field effect transistorinserted between the drain of the conversion use insulating gate typefield effect transistor and the data line and having a gate connected toa scanning line, a drive use insulating gate type field effecttransistor connected between a reference potential and a light emittingelement, a capacitor having a first electrode connected in common to agate of the conversion use insulating gate type field effect transistorand a gate of the drive use insulating gate type field effect transistorand having a second electrode connected to a reference potential, and aswitch use insulating gate type field effect transistor connectedbetween a gate and drain of said conversion use insulating gate typefield effect transistor and having a gate connected to a scanning line.100. A display device comprising a scanning line drive circuit forsuccessively selecting scanning lines, a data line drive circuitincluding a current source for generating a signal current having acurrent level in accordance with brightness information and successivelysupplying the same to data lines, and a plurality of pixels arranged atintersecting portions of the scanning lines and the data lines andincluding current driven type light emitting elements emitting light byreceiving the supply of the drive current, wherein each pixel comprisesa conversion use insulating gate type field effect transistor having asource connected to a reference potential, a fetch use insulating gatetype field effect transistor connected between the drain of theconversion use insulating gate type field effect transistor and the dataline and having a gate connected to a scanning line, a drive useinsulating gate type field effect transistor connected between areference potential and a light emitting element, a capacitor having afirst electrode connected to a gate of the drive use insulating gatetype field effect transistor and having a second electrode connected toa reference potential, and a switch use insulating gate type fieldeffect transistor connected between a gate of said conversion useinsulating gate type field effect transistor and a connecting pointbetween a gate of said drive use insulating gate type field effecttransistor and a first electrode of said capacitor and having a gateconnected to a scanning line.
 101. A display device as set forth inclaim 100, wherein the control terminal of the fetch use insulating gatetype field effect transistor and the control terminal of the switch useinsulating gate type field effect transistor are connected to differentscanning lines.
 102. A display device as set forth in claim 100, whereinthe size of the conversion use insulating gate type field effecttransistor is set larger than the size of the drive use insulating gatetype field effect transistor.
 103. A display device as set forth inclaim 101, wherein the switch use insulating gate type field effecttransistor is made conductive after a predetermined time within oneframe period after the switch use insulating gate type field effecttransistor and the fetch use insulating gate type field effecttransistor become nonconductive to extinguish in units of scanninglines.
 104. A pixel circuit for driving a current-driven type lightemitting element arranged at an intersecting portion of a data linesupplying a signal current of a current level in accordance withbrightness information and a scanning line supplying a selection pulseand emitting light by the drive current, comprising a receiving part forfetching the signal current from said data line in response to aselection pulse from said scanning line, a converting part forconverting a current level of the fetched signal current to a voltagelevel and holding the same, and a drive part for passing a drive currenthaving a current level in accordance with the held voltage level throughthe light emitting element.
 105. A pixel circuit as set forth in claim104, wherein the converting part includes a conversion use insulatinggate type field effect transistor provided with a gate, a source, adrain, and a channel and a capacitor connected to the gate.
 106. A pixelcircuit as set forth in claim 105, wherein the converting part includesa switch use insulating gate type field effect transistor insertedbetween the drain and the gate of the conversion use insulating gatetype field effect transistor, the switch use insulating gate type fieldeffect transistor becomes conductive when converting the current levelof the signal current to the voltage level and electrically connects thedrain and the gate of the conversion use insulating gate type fieldeffect transistor to create the voltage level with the source as thereference at the gate, and the switch use insulating gate type fieldeffect transistor is cut off and separates the gate of the conversionuse insulating gate type field effect transistor and the capacitorconnected to this from the drain when the capacitor holds the voltagelevel.
 107. A pixel circuit as set forth in claim 104, wherein: thereceiving part includes a fetch use insulating gate type field effecttransistor inserted between the drain of the conversion use insulatinggate type field effect transistor and the data line and the convertingpart includes a switch use insulating gate type field effect transistorinserted between the drain and the gate of the conversion use insulatinggate type field effect transistor.
 108. A pixel circuit as set forth inclaim 107, wherein the gate of the fetch use insulating gate type fieldeffect transistor and the gate of the switch use insulating gate typefield effect transistor are connected to different scanning lines. 109.A pixel circuit as set forth in claim 108, wherein the switch useinsulating gate type field effect transistor becomes conductive whenconverting the current level of the signal current to the voltage leveland electrically connects the drain and the gate of the conversion useinsulating gate type field effect transistor to create the voltage levelwith the source as the reference at the gate, the switch use insulatinggate type field effect transistor is cut off and separates the gate ofthe conversion use insulating gate type field effect transistor and thecapacitor connected to this from the drain when the capacitor holds thevoltage level, and the switch use insulating gate type field effecttransistor becomes unselected and is cut off before the fetch useinsulating gate type field effect transistor becomes nonconductive. 110.A pixel circuit as set forth in claim 109, wherein the switch useinsulating gate type field effect transistor is made conductive after apredetermined time within one frame period after the switch useinsulating gate type field effect transistor and the fetch useinsulating gate type field effect transistor become nonconductive toextinguish in units of scanning lines.
 111. A pixel circuit as set forthin claim 105, wherein a scanning line to which the switch use insulatinggate type field effect transistor is connected is provided independentlyfor each of the three primary colors.
 112. A pixel circuit as set forthin claim 107, wherein a conductivity type of said switch use insulatinggate type field effect transistor and a conductivity type of said fetchuse insulating gate type transistor are different.
 113. A pixel circuitas set forth in claim 105, wherein said drive part includes a drive useinsulating gate type field effect transistor provided with a gate, adrain, a source, and a channel, and the drive use insulating gate typefield effect transistor receives the voltage level held at the capacitorat its gate and passes a drive current having a current level inaccordance with that through the light emitting element via the channel.114. A pixel circuit as set forth in claim 113, wherein the gate of theconversion use insulating gate type field effect transistor and the gateof the drive use insulating gate type field effect transistor aredirectly connected to configure a current mirror circuit and wherein thecurrent level of the signal current and the current level of the drivecurrent are proportional.
 115. A pixel circuit as set forth in claim113, wherein the drive use insulating gate type field effect transistoris formed in the vicinity of the corresponding conversion use insulatinggate type field effect transistor inside the pixel and has an equivalentthreshold voltage to that of the conversion use insulating gate typefield effect transistor.
 116. A pixel circuit as set forth in claim 115,wherein the size of the conversion use insulating gate type field effecttransistor is set larger than the size of the drive use insulating gatetype field effect transistor.
 117. A pixel circuit as set forth in claim115, wherein the drive use insulating gate type field effect transistoroperates in the saturated region and passes a drive current inaccordance with a difference between the level of the voltage applied tothe gate thereof and the threshold voltage through the light emittingelement.
 118. A pixel circuit as set forth in claim 115, wherein thedrive use insulating gate type field effect transistors operates in thelinear region.
 119. A pixel circuit as set forth in claim 116, whereinthe drive use insulating gate type field effect transistors operates inthe linear region. 120-125. (canceled)
 126. A pixel circuit as set forthin claim 104, wherein the receiving part, the converting part, and thedrive part configure a current circuit combining a plurality ofinsulating gate type field effect transistors, and one or two or moreinsulating gate type field effect transistors have a double gatestructure for suppressing current leakage in the current circuit.
 127. Apixel circuit as set forth in claim 104, wherein the drive part includesan insulating gate type field effect transistor provided with a gate,drain, and a source and passes the drive current passing between thedrain and the source to the light emitting element in accordance withthe level of the voltage applied to the gate, and the light emittingelement is a two terminal type having an anode and a cathode, where thecathode is connected to the drain.
 128. A pixel circuit as set forth inclaim 104, wherein the drive part includes an insulating gate type fieldeffect transistor provided with a gate, a drain, and a source and passesa drive current passing between the drain and the source to the lightemitting element in accordance with the level of the voltage applied tothe gate, and the light emitting element is a two terminal type havingan anode and a cathode, where the anode is connected to the source. 129.A pixel circuit as set forth in claim 104, further including anadjusting means for downwardly adjusting the voltage level held by theconverting part and supplying the same to the drive part to tighten theblack level of the brightness of each pixel.
 130. A pixel circuit as setforth in claim 104, wherein a leak element is connected between saiddata line and a predetermined potential.
 131. A pixel circuit as setforth in claim 104, wherein an initial value setting element for settingsaid data to an initial value connected between said data line and apredetermined potential.
 132. A pixel circuit as set forth in claim 129,wherein the drive part includes an insulating gate type field effecttransistor having a gate, a drain, and a source, and the adjusting meansdownwardly adjusts the level of the voltage applied to the gate byraising the bottom of the voltage between the gate and the source of theinsulating gate type field effect transistor.
 133. A pixel circuit asset forth in claim 129, wherein the drive part includes an insulatinggate type field effect transistor having a gate, a drain, and a source,the converting part is provided with a capacitor connected to the gateof the thin film transistor and holding the voltage level, and theadjusting means comprises an additional capacitor connected to thatcapacitor and downwardly adjusts the level of the voltage to be appliedto the gate of the insulating gate type field effect transistor held atthat capacitor.
 134. A pixel circuit as set forth in claim 129, whereinthe drive part includes an insulating gate type field effect transistorhaving a gate, a drain, and a source, the converting part is providedwith a capacitor connected to the gate of the insulating gate type fieldeffect transistor on its one end and holding the voltage level, and theadjusting means adjusts the potential of the other end of the capacitorwhen holding the voltage level converted by the converting part at thatcapacitor to downwardly adjust the level of the voltage to be applied tothe gate of the insulating gate type field effect transistor.
 135. Apixel circuit as set forth in claim 104, wherein the light emittingelement comprises an organic electroluminescence element.
 136. A pixelcircuit as set forth in claim 113, wherein the drive use insulating gatetype field effect transistor comprises a P-channel type.
 137. A pixelcircuit for driving a current-driven type light emitting elementarranged at an intersecting portion of a data line supplying a signalcurrent of a current level in accordance with brightness information anda scanning line supplying a selection pulse and emitting light by thedrive current, comprising a conversion use insulating gate type fieldeffect transistor having a source connected to a reference potential, afetch use insulating gate type field effect transistor inserted betweenthe drain of the conversion use insulating gate type field effecttransistor and the data line and having a gate connected to a scanningline, a drive use insulating gate type field effect transistor connectedbetween a reference potential and a light emitting element, a capacitorhaving a first electrode connected in common to a gate of the conversionuse insulating gate type field effect transistor and a gate of the driveuse insulating gate type field effect transistor and having a secondelectrode connected to a reference potential, and a switch useinsulating gate type field affect transistor connected between a gateand drain of said conversion use insulating gate type field effecttransistor and having a gate connected to a scanning line.
 138. A pixelcircuit for driving a current-driven type light emitting elementarranged at an intersecting portion of a data line supplying a signalcurrent of a current level in accordance with brightness information anda scanning line supplying a selection pulse and emitting light by thedrive current, comprising a conversion use insulating gate type fieldeffect transistor having a source connected to a reference potential, afetch use insulating gate type field effect transistor connected betweenthe drain of the conversion use insulating gate type field effecttransistor and the data line and having a gate connected to a scanningline, a drive use insulating gate type field effect transistor connectedbetween a reference potential and a light emitting element, a capacitorhaving a first electrode connected to a gate of the drive use insulatinggate type field effect transistor and having a second electrodeconnected to a reference potential, and a switch use insulating gatetype field effect transistor connected between a gate of said conversionuse insulating gate type field effect transistor and a connecting pointbetween a gate of said drive use insulating gate type field effecttransistor and a first electrode of said capacitor and having a gateconnected to a scanning line.
 139. A pixel circuit as set forth in claim138, wherein the control terminal of the fetch use insulating gate typefield effect transistor and the control terminal of the switch useinsulating gate type field effect transistor are connected to differentscanning lines.
 140. A pixel circuit as set forth in claim 138, whereinthe size of the conversion use insulating gate type field effecttransistor is set larger than the size of the drive use insulating gatetype field effect transistor.
 141. A pixel circuit as set forth in claim139, wherein the switch use insulating gate type field effect is madeconductive after a predetermined time within one frame period after theswitch use insulating gate type field effect transistor and the fetchuse insulating gate type field effect transistor become nonconductive toextinguish in units of scanning lines.
 142. A method of driving a lightemitting element for driving a current-driven type light emittingelement arranged at an intersecting portion of a data line supplying asignal current of a current level in accordance with brightnessinformation and a scanning line supplying a selection pulse and emittinglight by the drive current, comprising a receiving routine for fetchingthe signal current from said data line in response to a selection pulsefrom said scanning line, a converting routine for converting a currentlevel of the fetched signal current to a voltage level and holding thesame, and a drive routine for passing a drive current having a currentlevel in accordance with the held voltage level through the lightemitting element.
 143. A method of driving a light emitting element asset forth in claim 142, wherein the converting routine includes aroutine using a conversion use insulating gate type field effecttransistor provided with a gate, a source, a drain, and a channel and acapacitor connected to the gate, in the routine, the conversion useinsulating gate type field effect transistor creates the voltage levelconverted by passing the fetched signal current though the channel inthe receiving routine at the gate, and the capacitor holds voltage levelcreated at the gate.
 144. A method of driving a light emitting elementas set forth in claim 143, wherein the converting routine includes aroutine using a switch use insulating gate type field effect transistorinserted between the drain and the gate of the conversion use insulatinggate type field effect transistor, in the routine, the switch useinsulating gate type field effect transistor becomes conductive whenconverting the current level of the signal current to the voltage leveland electrically connects the drain and the gate of the conversion useinsulating gate type field effect transistor to create the voltage levelwith the source as the reference at the gate, and the switch useinsulating gate type field effect transistor is cut off and separatesthe gate of the conversion use insulating gate type field effecttransistor and the capacitor connected to this from the drain when thecapacitor holds the voltage level.
 145. A method of driving a lightemitting element as set forth in claim 143, wherein: said drive routinesincludes a routine using a drive use insulating gate type field effecttransistor provided with a gate, a drain, a source, and a channel, andin the routine, the drive use insulating gate type field effecttransistor receives the voltage level held at the capacitor at its gateand passes a drive current having a current level in accordance withthat through the light emitting element via the channel.
 146. A methodof driving a light emitting element as set forth in claim 145, whereinthe gate of the conversion use insulating gate type field effecttransistor and the gate of the drive use insulating gate type fieldeffect transistor are directly connected to configure a current mirrorcircuit and wherein the current level of the signal current and thecurrent level of the drive current are proportional.
 147. A method ofdriving a light emitting element as set forth in claim 145, wherein thedrive use insulating gate type field effect transistor is formed in thevicinity of the corresponding conversion use insulating gate type fieldeffect transistor inside the pixel and has an equivalent thresholdvoltage to that of the conversion use insulating gate type field effecttransistor.
 148. A method of driving a light emitting element as setforth in claim 147, wherein the drive use insulating gate type fieldeffect transistor operates in the saturated region and passes a drivecurrent in accordance with a difference between the level of the voltageapplied to the gate thereof and the threshold voltage through the lightemitting element. 149-154. (canceled)
 155. A method of driving a lightemitting element as set forth in claim 143, wherein the receivingroutine, the converting routine, and the drive routine are executed on acurrent circuit combining a plurality of insulating gate type fieldeffect transistors, and one or two or more insulating gate type fieldeffect transistors have a double gate structure for suppressing currentleakage in the current circuit.
 156. A method of driving a lightemitting element as set forth in claim 142, wherein the drive routine isperformed using an insulating gate type field effect transistor providedwith a gate, drain, and a source and passes the drive current passingbetween the drain and the source to the light emitting element inaccordance with the level of the voltage applied to the gate, and thelight emitting element is a two terminal type having an anode and acathode, where the cathode is connected to the drain.
 157. A method ofdriving a light emitting element as set forth in claim 142, wherein thedrive routine is performed using an insulating gate type field effecttransistor provided with a gate, a drain, and a source and passes adrive current passing between the drain and the source to the lightemitting element in accordance with the level of the voltage applied tothe gate, and the light emitting element is a two terminal type havingan anode and a cathode, where the anode is connected to the source. 158.A method of driving a light emitting element as set forth in claim 142,further including an adjusting routine for downwardly adjusting thevoltage level held by the converting routine and supplying the same tothe drive part to tighten the black level of the brightness of eachpixel.
 159. A method of driving a light emitting element as set forth inclaim 158, wherein the drive routine includes uses an insulating gatetype field effect transistor having a gate, a drain, and a source, andthe adjusting routine downwardly adjusts the level of the voltageapplied to the gate by raising the bottom of the voltage between thegate and the source of the insulating gate type field effect transistor.160. A method of driving a light emitting element as set forth in claim158, wherein the drive routine uses an insulating gate type field effecttransistor having a gate, a drain, and a source, the converting routineuses a capacitor connected to the gate of the thin film transistor andholding the voltage level, and the adjusting routine uses an additionalcapacitor connected to that capacitor and downwardly adjusts the levelof the voltage to be applied to the gate of the insulating gate typefield effect transistor held at that capacitor.
 161. A method of drivinga light emitting element as set forth in claim 158, wherein the driveroutine uses an insulating gate type field effect transistor having agate, a drain, and a source, the converting routine uses a capacitorconnected to the gate of the insulating gate type field effecttransistor on its one end and holding the voltage level, and theadjusting means routine adjusts the potential of the other end of thecapacitor when holding the voltage level converted by the convertingroutine at that capacitor to downwardly adjust the level of the voltageto be applied to the gate of the insulating gate type field effecttransistor.
 162. A method of driving a light emitting element as setforth in claim 142, wherein the light emitting element comprises anorganic electroluminescence element.
 163. A display device including:scanning lines for selecting pixels and data lines giving brightnessinformation for driving the pixels arranged in a matrix, each pixelincluding a light emitting element changing in brightness by an amountof current supplied, a writing means controlled by a scanning line andwriting the pixel brightness information given from the data line, and adrive means for controlling the amount of current supplied to said lightemitting element in accordance with the written brightness information,the brightness information being written in each pixel by applying anelectric signal in accordance with the brightness information to thedata line in the state with the scanning line selected, the brightnessinformation written in each pixel being held in each pixel even afterthe scanning line is not selected and the light emitting element of eachpixel able to remain lighted by a brightness in accordance with the heldbrightness information, further comprising an adjusting means fordownwardly adjusting the brightness information written by said writingmeans an supplying the same to said drive means to tighten the blacknesslevel of each pixel.
 164. A pixel circuit for driving a pixel having alight emitting element arranged at an intersecting portion of a dataline supplying brightness information and a scanning line supplying aselection pulse and emitting light in accordance with said brightnessinformation, including a writing means controlled by a scanning line andwriting in the pixel brightness information given from the data line anda drive means for controlling the amount of current supplied to saidlight emitting element in accordance with the written brightnessinformation, the brightness information being written in each pixel byapplying an electric signal in accordance with the brightnessinformation to the data line in the state with the scanning lineselected, the brightness information written in each pixel being held ineach pixel even after the scanning line is not selected and the lightemitting element of each pixel able to remain lighted by a brightness inaccordance with the held brightness information, further comprising anadjusting means for downwardly adjusting the brightness informationwritten by said writing means and supplying the same to said drive meansto tighten the blackness level of each pixel.
 165. A method of driving adisplay device including scanning lines for selecting pixels and datalines giving brightness information for driving the pixels arranged in amatrix, each pixel including a light emitting element changing inbrightness by an amount of current supplied, comprising: a writingroutine controlled by a scanning line and writing in the pixelbrightness information given from the data line and a drive routine forcontrolling the amount of current supplied to said light emittingelement in accordance with the written brightness information, thebrightness information being written in each pixel by applying anelectric signal in accordance with the brightness information to thedata line in the state with the scanning line selected, the brightnessinformation written in each pixel being held in each pixel even afterthe scanning line is not selected and the light emitting element of eachpixel able to remain lighted by a brightness in accordance with the heldbrightness information, further comprising an adjusting routine fordownwardly adjusting the brightness information written by said writingroutine and supplying the same to said drive routine to tighten theblackness level of each pixel.